Executive Summary

The correct order in AGV selection is to fix the drive configuration first - differential drive, steer drive or Mecanum - and only then to specify the wheels. The configuration determines how many wheels the vehicle needs, the direction in which each wheel is loaded, how the axle load is distributed, and which failure mode will appear first. Skipping that step and comparing wheel parameters directly is a selection made while the load conditions are still undefined.

Differential drive is structurally simple and cost-effective. It steers through the speed difference between the left and right drive wheels, which means the accuracy of the whole system is tied directly to the diameter consistency of those two wheels. A small diameter difference is amplified by the differential mechanism into a heading error. On a theoretical basis, with a wheel spacing of 600 mm and a 10 m straight run, a 0.5 mm difference between the left and right wheel diameters corresponds to a lateral deviation of about 208 mm.

Steer drive integrates traction and steering into a single slewing unit. It offers high manoeuvrability, lateral travel and in-place rotation, and the price is paid at the contact patch: during a turn, the contact patch scrubs sideways around the slewing axis. A steer unit with a 30 mm offset scrubs the tread roughly 47 mm across the floor for every 90-degree turn it completes.

The omnidirectional capability of a Mecanum wheel comes from controlled sliding between the rollers and the floor. Energy efficiency is lower than with a conventional wheel set, and the demands on floor flatness, load distribution and maintenance rhythm are noticeably higher. All of these costs need to be brought into the evaluation rather than discovered later.

Whatever the configuration, the demands it makes on the wheels can be broken down into four universal indicators: diameter consistency, tread friction and wear orientation, hub concentricity and interface accuracy, and the bearing and load-carrying arrangement. These four indicators carry different weights under the three configurations, and the ranking is what a wheel specification has to reflect.

HANKE's GSFH, GSFM and other flange-series drive wheels cover a diameter range of 100 mm to 300 mm and a single-wheel load range of 300 kg to 1,100 kg at 4 km/h, and can be matched to the mounting requirements of steer unit slewing assemblies and differential drive units. HANKE's Eamflex 93A high-wear-resistance tread and Saxflex 75A floor-protection tread correspond respectively to the traction-first and floor-protection-first orientations.

Key Differences Between the Three Drive Configurations at a Glance

1. Why the Drive Configuration Must Be Fixed Before the Wheels

On an AGV project, the wheels are usually the last parts to be discussed. The navigation concept, the fleet scheduling logic and the structural form of the vehicle are all settled first, and the wheels are purchased at the end as the executing components. That order is not wrong in itself. The problem is that many projects treat the wheel as a standard purchased part: pick a diameter from the load, pick a hardness from experience, fit it and move on.

The consequence appears only after the vehicle is running. Drifting, out-of-tolerance stopping positions and uneven tread wear show up, and the investigation is usually pointed at navigation and algorithms - while the real source sits in the match between the drive configuration and the wheel. Three transmission chains explain how that match is created.

1.1 Chain One: The Drive Configuration Defines the Direction of the Loads on the Wheel

A polyurethane wheel is not the same component in every configuration. Under differential drive, the follower wheels carry a following lateral force: they are dragged along by the body and their tread is scrubbed sideways across the floor during a turn. Under steer drive, the drive wheel carries traction and vertical load plus the steering reaction torque transmitted through the slewing unit, and it is scrubbed laterally at the moment of steering. Under a Mecanum layout, the rollers carry radial pressure and axial force components at the same time, so the load state is a composite one.

Different load directions call for different compound orientations. Duties dominated by traction require a stable friction coefficient and good wear resistance, and a harder, more wear-resistant compound suits them. Duties dominated by being dragged and scrubbed require the tread to leave as little marking on the floor as possible and to generate as little heat as possible, which calls for a softer compound and lower rolling resistance. HANKE's Eamflex 93A tread system is oriented towards high wear resistance, and the Saxflex 75A tread system towards floor protection. The two orientations correspond to the two load states above, and choosing the wrong orientation tends to reveal itself only after several hundred hours of operation.

1.2 Chain Two: The Drive Configuration Defines Axle Load Distribution and Load Transfer

Estimating the single-wheel load from a static, evenly divided value is a very common simplification at the selection stage. A four-wheel layout with a vehicle plus payload of 4,000 kg gives 1,000 kg per wheel statically, which looks sufficient. But an AGV transfers load between its wheels during start-up, braking and turning, and the amount of transfer can be estimated with a simple expression.

The load transfer is dF = m x a x h / B, where m is the total vehicle mass in kilograms, a is the relevant acceleration, deceleration or lateral acceleration in metres per second squared, h is the height of the centre of gravity in metres, and B is the wheel spacing in metres. Substituting a common set of values - m of 4,000 kg, a of 0.5, h of 0.5 m and B of 0.8 m - gives dF = 4,000 x 0.5 x 0.5 / 0.8 = 1,250 N, equivalent to about 127 kgf of vertical load transferring from the inner wheel to the outer wheel.

The point of that number is this: under dynamic conditions the actual load on a single wheel can be one to two tenths higher than the static evenly divided value. If the margin allowed at selection was already tight, then turning, climbing and emergency braking are the moments when the problem concentrates and becomes visible. The exact increment that lands on each wheel depends on the wheel count and the suspension arrangement and has to be calculated item by item from the vehicle parameters - but the direction is clear. The closer a configuration is to omnidirectional motion and the more frequently it turns, the less it can afford to skip the dynamic load.

A single-steer-wheel layout is a typical example. The vehicle has only one driven point, carrying both steering and traction, so the load requirement on that wheel is far above the value obtained by dividing the total by the number of wheels. The accuracy requirement on the flange interface is also stricter, because any deflection of the slewing unit translates directly into an error in the vehicle attitude.

1.3 Chain Three: The Drive Configuration Decides Which Wheel Becomes the Bottleneck

The wear and accuracy bottlenecks of the three configurations do not fall in the same place. Under differential drive the bottleneck is the diameter consistency of the left and right drive wheels. The differential controller assumes that both sides have the same diameter; once a real difference exists, the same speed command corresponds to different actual linear speeds and the heading turns with it.

Under steer drive the bottleneck is the concentricity of the flange interface plus steering scrub wear. Runout of the mounting face of the slewing unit is amplified into an oscillation of vehicle attitude, while long-term lateral scrubbing of the tread produces an uneven wear band. Under a Mecanum layout the bottleneck is the roller itself. The roller is an independent component covered in rubber or polyurethane, carrying load and sliding friction at the same time, and it wears faster than a conventional tread. More than ten rollers spread across four wheels wearing at different rates shows up as an error in direction and displacement during omnidirectional motion.

Put the three chains together and it becomes clear why the order of selection cannot be reversed. Only once the configuration is fixed can the direction of the loads, the number of wheels and the dynamic load be fixed, and only then can it be decided which indicator deserves the extra margin. Choosing wheel parameters first is a local optimisation carried out in a load environment that has not yet been defined. This is also the reasoning behind the per-wheel-position compound recommendation that HANKE applies in AGV projects rather than a single tread specification for the whole vehicle.

2. Differential Drive: Simple Structure, High Sensitivity to Diameter Consistency

2.1 Structure and Kinematic Characteristics

Differential drive is one of the most widely used AGV configurations: one independent drive wheel on each side, driven directly by its own motor or through a gearbox, with two to four follower or castor wheels elsewhere on the body. The motion of the vehicle is determined by the speed difference between the two sides, and the kinematic relationships can be written as three expressions.

Take a vehicle with a wheel spacing of 800 mm. With the right wheel at 0.6 m/s and the left at 0.2 m/s, the body linear speed is 0.4 m/s, the angular speed is 0.5 rad/s and the turning radius is about 0.8 m. When the two sides run at equal speed in opposite directions, the vehicle rotates in place around the midpoint between the two drive wheels, with a turning radius of about half the wheel spacing. That characteristic makes path planning relatively simple, but it also means the turning centre can only fall on the axis of the two drive wheels.

2.2 Wheel Set Configuration and Load Analysis

The division of labour between the two kinds of wheel in a differential wheel set is clear: the drive wheels provide traction and positioning, and the follower wheels provide support and follow the body. On a straight run the lower the rolling resistance of a follower wheel the better, but in a turn it is dragged sideways by the body and its tread is scrubbed across the floor. This is the direct reason why, on differential-drive vehicles, follower wheel tread wear is often more irregular than drive wheel wear.

These two kinds of wheel should not be specified with one set of parameters. Fitting the wear-resistant drive wheel compound to a follower wheel buys higher rolling resistance and a greater risk of scratching the floor.

2.3 Four Requirements on the Wheels

First, diameter consistency is the primary sensitivity of differential drive. The differential controller sends the same speed command to both sides, but the actual displacement is determined by the actual diameter of each wheel. The deviation accumulates with every revolution and grows with the square of the distance travelled. The lateral deviation can be estimated with a simplified expression: lateral deviation approximately equals L squared times (dD / D) divided by (2 x B), where L is the straight-run distance, dD is the left/right diameter difference, D is the wheel diameter and B is the wheel spacing.

Calculation assumptions: wheel diameter D of 200 mm, wheel spacing B of 600 mm, the two wheels on the same side having identical diameters, both sides receiving the same speed command, and closed-loop controller correction and floor slip ignored. A real vehicle with closed-loop correction will visibly weaken the accumulation effect, but the correction has to be paid for with extra speed on one side, which wears that tread faster and, over long operation, widens the diameter difference again. What this set of numbers demonstrates is not any particular value but an order of magnitude: a diameter difference on the scale of a few tenths of a millimetre already leaves a lateral deviation of ten to a few tens of centimetres within a 10 m run.

Second, tread friction and wear orientation have to be set from the actual load. A drive wheel must transmit traction while turning, so its friction coefficient has to be stable; a follower wheel is continuously dragged sideways, so a high-friction compound would raise rolling resistance and floor wear. In practice this pair of conflicting demands is resolved by specifying the parts separately: drive wheels biased towards wear resistance and stable friction, follower wheels biased towards low resistance and floor protection. Taking HANKE's Eamflex 93A and Saxflex 75A tread systems as an example, selecting by wheel position on the same vehicle is a fairly common practice in AGV wheel set configuration.

Third, hub concentricity and radial runout determine whether the odometer can be trusted. The distance travelled by an AGV is usually derived from the drive wheel encoder, and if the effective rolling radius of the wheel fluctuates because of runout, the calculated displacement carries a systematic error. This item is secured through factory inspection and batch records.

Fourth, the bearing and load-carrying arrangement has to cover both the radial and the axial direction. A drive wheel experiences radial load and axial thrust simultaneously during start-stop and turning, so the bearing combination has to be chosen from the vehicle tare weight, the rated load, the start-stop frequency and the turning radius rather than from wheel diameter alone.

2.4 Typical Failure Modes Under Differential Drive

The four symptoms in the table are arranged in the order they usually appear on site. Drifting is reported first, odometry error is noticed when docking failure rates rise, and the tread and floor symptoms are found later during routine inspection or when a customer complains about marks on the floor.

3. Steer Drive: Excellent Manoeuvrability, Paid For in Steering Scrub

3.1 Structure and Kinematic Characteristics

A steer wheel integrates the drive motor, the gearbox, the slewing unit and the drive wheel into one assembly. The wheel can be driven to rotate and can also be deflected as a whole together with the slewing unit. By the number of steer units, the layouts divide into single-steer, dual-steer and four-steer arrangements: a single steer wheel combined with several follower wheels delivers a wide range of manoeuvrability, while a four-steer arrangement can achieve lateral travel, crab travel and in-place rotation and is a common choice where space is constrained.

From the wheel's point of view, the mounting interface of a steer layout is very different from that of a differential layout. The steer drive wheel has to be matched to the slewing unit and the gearbox through a flange structure, and the accuracy of the hub mating face, the bolt hole positions and the concentricity with the slewing axis all affect the stability of the vehicle attitude. In industrial practice this type of mounting usually calls for a flange-structure drive wheel. HANKE's GSFH, GSFM and other flange series cover a diameter range of 100 mm to 300 mm and a single-wheel load range of 300 kg to 1,100 kg at 4 km/h, and can be matched item by item to the steering unit interface.

3.2 The Geometric Relationship Between Offset and Steering Scrub

When a steer wheel turns, the contact patch does not rotate about its own centre - it revolves around the slewing axis. The distance from the slewing axis to the centre of the contact patch is called the offset, and the larger the offset, the longer the distance the tread scrubs sideways across the floor for each completed steering movement. The arc length can be calculated with a simple geometric expression: s = e x theta, where e is the offset and theta is the turn angle in radians, with 90 degrees equal to about 1.571 radians.

This set of numbers explains two things. First, why a steer drive layout landing in a cleanroom, a paint shop, or a pharmaceutical or food workshop needs extra attention to the marking and particle generation caused by tread scrub - the steering action itself is creating lateral friction. Second, why tread wear on steer-drive vehicles is usually distributed as a band rather than as uniform wear around the full circumference: the scrub is concentrated in the lateral direction of the contact patch.

There are three engineering responses. Reduce the offset so that the contact patch sits closer to the slewing axis. Separate the steering movement from the travel movement in time, so that the vehicle does not steer and travel at the same moment. And make a deliberate choice in the tread compound: on the same vehicle, a steer wheel position that turns frequently and a follower wheel position that only supports load should not be using the same compound in the first place. HANKE approaches the third of these by recommending a compound for each wheel position rather than one compound for the whole vehicle.

3.3 Four Requirements on the Wheels

First, the concentricity and face runout of the flange interface. Any mounting error between the slewing unit and the wheel hub is added directly to the vehicle attitude. This is an assembly accuracy item and has to be secured jointly at the hub machining stage and at the finished-product inspection stage.

Second, tread wear performance and dimensional stability after scrubbing. The tread of a steer wheel is subjected to lateral scrubbing over a long period and wears faster than under pure rolling conditions, so indicators such as abrasion volume and wear resistance grade matter more. The DIN 53516 abrasion test method is commonly referenced in the industry for comparison.

Third, lateral load-carrying capacity matched to the slewing unit. A steer wheel transmits lateral force to the flange and the slewing bearing while turning, and insufficient structural rigidity in the hub shows up as a slight rocking of the vehicle during steering - a symptom that is easily misdiagnosed as a parameter problem during commissioning.

Fourth, diameter and friction consistency across the multiple steer wheels on one vehicle. In a four-steer arrangement, if the diameter or friction characteristic of any one wheel differs from the other three, that difference is converted into a direction error during coordinated motion, so pairing and batch consistency matter just as much in this layout.

3.4 Typical Failure Modes Under Steer Drive

Note that three of the four symptoms point back to the interface or the tread band rather than to the wheel as a whole. On steer-drive vehicles, the wheel is rarely the weak link on its own; it is the joint between the wheel and the slewing unit that concentrates the errors.

4. Mecanum Wheels: Omnidirectional Capability Comes from Sliding, and the Cost Has to Be Counted

4.1 Structure and Kinematic Characteristics

A Mecanum wheel carries a ring of freely rotating rollers mounted diagonally around the circumference of each wheel, with the roller axis typically set at 45 degrees to the wheel plane. With four wheels mounted in fixed orientations, a linear combination of the four wheel speeds resolves into three motion components for the body - longitudinal, lateral and yaw. The layout therefore achieves lateral travel and crab travel without any steering mechanism, which is why it is adopted in narrow aisles and at stations that need frequent repositioning.

In mechanism terms, the omnidirectional capability of a Mecanum wheel is not obtained by steering the wheel. It is produced by the continuous relative sliding between the rollers and the floor. That single fact determines its efficiency and wear behaviour, and it also determines how particular the layout is about its operating conditions.

4.2 Requirements on the Rollers, the Hub and the Floor

The last two items are the ones most often overlooked. A Mecanum layout converts a floor condition that a conventional wheel set would tolerate into a positioning error, and it converts an uneven load distribution into a difference in roller wear rate - which then feeds back into the direction of travel.

4.3 Efficiency and Wear Cost

The energy efficiency of a Mecanum wheel is lower than that of a conventional wheel set, because sliding friction exists between the rollers and the floor and part of the input energy is consumed in sliding and in the internal rotation of the rollers. The exact loss ratio depends on the roller material, the floor friction characteristics, the load and the motion pattern, and it varies considerably between duties, so it is not appropriate to summarise it with a single fixed figure. What can be stated is a direction: under the same load and path conditions, the drive power requirement of a Mecanum layout is usually higher than that of a differential or steer layout, and that has to be taken into account when the selection is costed.

On wear, the roller is both a load-carrying part and a friction part, and it wears faster than a conventional tread. Uneven wear among the several rollers on one wheel introduces a deviation into the resultant direction of omnidirectional motion. The maintenance rhythm for a Mecanum layout is therefore usually denser: periodic checks of roller wear, freedom of roller rotation and wear consistency across the four wheels matter more than they do on a conventional wheel set.

Objectively, Mecanum is not a layout to be avoided - it is a layout whose costs have to be counted properly. At stations where space is constrained and lateral repositioning is frequent, the efficiency gain it delivers is real. On long-distance, high-load duties with ordinary floor conditions, a conventional wheel set usually performs better overall.

4.4 Typical Failure Modes of a Mecanum Wheel System

These four symptoms have one thing in common: in each case, part of the cause lies with the environment and with the motion pattern rather than with the wheel. That is consistent with the character of the layout itself. A Mecanum system is more sensitive to how it is used than a conventional one.

5. A Multi-Dimensional Comparison of the Three Drive Configurations

The differences between the three configurations are collected into four tables so that the direction can be located quickly at an early project stage. Each table is followed by a single conclusion that can be used directly in a selection discussion.

Table 1: Kinematics and Structure

Conclusion: the higher the demand for path flexibility, the higher the structural complexity, and part of the cost of that complexity transfers to the mounting accuracy and consistency requirements on the wheels.

Table 2: Comparison of What Each Configuration Demands from the Wheels

Conclusion: the four wheel indicators carry different weights under different configurations. At selection, the priority order should be set by the configuration rather than by copying one set of parameters across all three.

Table 3: Comparison of Suitability for Operating Conditions and Floors

Conclusion: the poorer the floor condition, the higher the load and the longer the travel distance, the more the value of a conventional wheel set stands out. The stronger the manoeuvrability requirement and the tighter the space, the more an omnidirectional layout earns its place.

Table 4: Comparison of Failure Modes and Maintenance Focus

Conclusion: the maintenance strategy should follow the configuration. The inspection items and cycles differ. Managing all three configurations with a single maintenance sheet leaves each one's sensitive items unchecked.

6. A Seven-Step Selection Workflow from Drive Configuration to Wheel

The logic of the previous chapters is condensed below into an executable sequence. The seven steps can be worked through over several discussions early in a project, but the order should not be reversed: the output of each step is the input to the next.

Step 1: establish the motion degrees of freedom required. Answer three questions first - is lateral travel needed, is in-place rotation needed, and is there a hard upper limit on the turning radius. If the first two answers are both no, differential drive is usually sufficient. If either of them is mandatory, the comparison moves to steer drive or Mecanum.

Step 2: establish the floor conditions and the path characteristics. Record the floor type, flatness, joints and slope, cleanliness requirements, the presence of oil or water, and the typical travel distance and turning frequency of a single task. This group of information determines both whether a configuration is feasible and which way the tread compound should lean.

Step 3: calculate the axle load, including dynamic load transfer. Use dF = m x a x h / B to estimate the transfer, add the dynamic increment on top of the static evenly divided value to obtain the single-wheel load requirement used for selection, and state the speed condition at the same time. Load capacity must always be given together with the speed condition - for example, the single-wheel load at 4 km/h - because a load figure divorced from its speed condition cannot be compared with anything.

Step 4: establish the wheel diameter and the wheel width. Diameter affects obstacle crossing, rolling resistance and load capacity; width affects ground contact pressure and lateral stability. On heavy-load duties, increasing the wheel diameter or width moderately is more effective at reducing the pressure per unit area than simply raising the tread hardness.

Step 5: establish the tread hardness and the compound orientation. A wheel position that prioritises traction takes a harder, wear-resistant orientation; a position that is passively dragged and prioritises floor protection takes a softer, low-resistance orientation; a position that is scrubbed frequently during steering strikes a balance between wear resistance and scrub resistance. HANKE normally gives compound recommendations by wheel position at this stage rather than applying a single tread to the whole vehicle.

Step 6: establish the hub structure and the bearing arrangement. The hub material, surface treatment, flange or keyway interface, bearing type and clearance grade all have to be matched to the drive configuration, the gearbox and the vehicle load spectrum. Bearing selection and hub structure should be considered together; deciding them separately usually leads to rework at the assembly stage.

Step 7: establish the pairing rules and the acceptance criteria. State the permissible range of the diameter difference between the left and right wheels on one axle, the consistency requirement across multiple wheels on one vehicle, and which data has to be checked at acceptance. This step turns the conclusions of the previous six into a technical agreement that can be delivered and verified.

7. Purchasing and Acceptance: Writing Configuration Compatibility into the Technical Agreement

The logic of the previous chapters ultimately has to land on a written technical agreement. Many drifting, abnormal noise and early wear problems do not originate in manufacturing quality. They originate in the purchasing stage, where the compatibility between the configuration and the wheel was never written down clearly, so the supplier delivers to a generic part number and the integrator accepts against a generic part number - and neither of them lands on the critical items.

The earlier this agreement is written clearly, the more rework can be avoided later at the assembly and commissioning stages. The items are not complicated; they simply need to be present before the purchase order is issued rather than reconstructed afterwards.

8. How HANKE Approaches It: Moving the Configuration's Precision Requirements Upstream into the Process

8.1 One Premise

Errors from the drive configuration all end up at the wheel. The left/right diameter difference of a differential drive, the flange interface runout of a steer drive and the roller wear consistency of a Mecanum layout all present themselves as vehicle-level symptoms, but their origin lies in the machining and inspection stages of the wheel. Problems of this kind share one characteristic: they are difficult to compensate for after the fact and controllable beforehand. Once the wheels are on the vehicle, the differences between them can only be absorbed by repeated calibration on the vehicle side, at a cost far higher than controlling them at the manufacturing end. For HANKE this premise is what determines where the control points are placed in the process.

8.2 From the Symptom the Customer Sees Back to the Source Process

This table is the core of the section. It maps the symptoms a customer sees on the vehicle, item by item, to a specific process, a control action and a piece of data that can be checked. Its value is not in the adjectives - it is that every line can be verified.

8.3 Matching Points for the Three Configurations

One point needs to be made explicitly. The roller assembly of a Mecanum wheel is a non-standard structural part, and its selection and prototyping have to be discussed and confirmed item by item with the supplier against the drawing. It should not be ordered by applying a conventional wheel parameter table. That is also why the matching capability for the first three configurations is described in detail here while the Mecanum layout is listed separately.

8.4 What the Customer Can Obtain

8.5 One Thing That Should Be Said Clearly

No manufacturing system should promise that a wheel will never deviate. That is neither realistic nor consistent with engineering practice. What deserves more attention is something else: whether the deviation is held inside a range that is predictable, verifiable and traceable. When a wheel behaves abnormally, can the inspection records for that batch be retrieved, can the specific process and the parameter that drifted be identified, and can a corrective action be given. That is the substance of capability management.

Take the supply record for the conveyor system on a Mercedes-Benz production line as an example: over a 48-month operating period there were no wheel-related after-sales issues. A record of that kind does not demonstrate that problems never occur. It demonstrates that when precision is managed continuously as a process indicator, long-term stability is achievable. HANKE's AGV servo wheel project for Kinco likewise centred on diameter consistency and batch data management. The two projects involve different duties and different configurations, but the requirement on wheel consistency is the same in both.

8.6 Operating-Condition Factors Worth Confirming Item by Item at the Selection Stage

9. Frequently Asked Questions

Q1: When selecting an AGV, should the drive configuration or the wheels be decided first?

The drive configuration. The configuration determines the number of wheels, the direction of the loads, the dynamic load and the failure mode. Only after the configuration is fixed do wheel diameter, tread orientation, hub structure and pairing rules have a definite basis for choice. Picking wheel parameters first and then assembling a configuration around them is a local optimisation carried out while the load conditions are still undefined, and problems tend to surface during vehicle commissioning.

Q2: What is the main difference between differential drive and steer drive?

The main difference lies in manoeuvrability and structural cost. Differential drive steers through the speed difference between the left and right wheels, so its turning centre can only fall on the axis of the two drive wheels and it cannot travel laterally. Steer drive deflects actively through a slewing unit, so it can travel laterally and diagonally and offers better manoeuvrability. The cost is a more complex structure, a more concentrated single-point load, and lateral scrub of the tread around the slewing axis during steering, which raises the requirements on interface accuracy and tread orientation.

Q3: Why is differential drive so sensitive to the diameter difference between the left and right wheels?

Because the differential controller sends the same speed command to both sides, while the actual displacement is determined by the actual diameter of each wheel. The diameter difference accumulates into a heading error with every revolution and grows with the square of the distance travelled. On a theoretical basis, with a wheel diameter of 200 mm, a wheel spacing of 600 mm and a left/right diameter difference of 0.5 mm, the lateral deviation over a 10 m straight run is about 208 mm.

Q4: How large is the lateral scrub when a steer wheel turns?

The scrub displacement depends on the offset from the slewing axis to the centre of the contact patch, and the geometric relationship is arc length equal to offset multiplied by turn angle in radians. With an offset of 30 mm, the scrub displacement for a single 90-degree turn is about 47 mm, and the accumulation over a full 360 degrees is about 188 mm. This is also why steer drive layouts need extra attention to tread orientation in clean and marking-sensitive applications.

Q5: Can Mecanum wheels replace a conventional AGV wheel set?

Not as a straightforward substitution - the two have different application conditions. The advantage of a Mecanum wheel is that it achieves lateral and diagonal travel without a steering mechanism, which suits space-constrained stations that need frequent repositioning. The cost is lower energy efficiency, a high requirement on floor flatness, and rollers that wear faster than a conventional tread. On long-distance, high-load duties with ordinary floor conditions, a conventional wheel set usually performs better overall.

Q6: Can the same tread be used for the drive wheels and the follower wheels on one AGV?

Not recommended. A drive wheel mainly transmits traction and needs a stable friction coefficient and good wear resistance; a follower wheel mainly supports and follows and needs low rolling resistance and floor protection. Using one compound for both means one of the two requirements has to be sacrificed - either the follower wheels raise the energy consumption of the whole vehicle and scratch the floor, or the drive wheels do not deliver enough traction. Selecting the compound by wheel position is the more common practice.

Q7: How should the load capacity of a wheel be specified?

Do not give a load figure on its own; always give the speed condition with it. The load capacity of the same wheel differs at different speeds, and a load figure divorced from its speed condition cannot be compared. The method is to obtain a base value from the static evenly divided load, add the dynamic increment using the load transfer expression, and finally state the rated speed and the overspeed condition that may occur.

Q8: The AGV starts drifting after running for a while. Where should the investigation start?

Work from the easy checks to the harder ones. Start with a visual and fastener inspection, covering the mounting condition of the wheels and the tightness of the flange or shaft. Then take multi-point diameter measurements of the left and right drive wheels and record them. Next check radial runout and tread wear uniformity. Finally re-verify whether the hardness and friction characteristics have drifted. Keeping the diameter data on file is very helpful for judging the trend later.

Q9: What special requirements apply to AGV wheels in cleanrooms and paint shops?

Two things matter most. The first is the particles and marking generated by tread wear and steering scrub. The second is the interaction between the compound and the floor. Practical measures include reducing unnecessary steering movements, giving preference to a floor-protection tread orientation, and separating the positions that turn frequently from the positions that only support load when the wheel set is configured.

Q10: How can you judge whether a wheel supplier's capability is reliable?

Look at four things. Whether batch key-dimension inspection records exist. Whether the critical processes have controlled specifications with upper and lower parameter limits. Whether the supplier can assess the vehicle against its operating-condition factors and give a per-position recommendation. And whether an abnormal finding can be traced back to a specific batch and process. HANKE addresses these four with batch inspection archives, process specifications, operating-condition assessment and batch traceability, and customers can request the corresponding data at the selection stage.

10. Summary and Recommendations

The seven recommendations below can be used directly in a selection discussion for an AGV project.

First, fix the order of selection. Establish the required motion degrees of freedom first, then the drive configuration, and only then the wheel parameters. Reversing the order produces local optimisation and system-level rework.

Second, let the configuration set the sensitive items. Differential drive is governed by left/right diameter consistency, steer drive by flange interface accuracy and steering scrub, and a Mecanum layout by roller wear consistency and floor condition. Different sensitive items mean different inspection priorities.

Third, never state a load requirement without its speed condition. Start from the static evenly divided load, add the dynamic increment using the load transfer expression, and write both figures into the technical agreement.

Fourth, configure one vehicle by wheel position. Drive wheels, follower wheels, guide wheels and steer wheels each have a different orientation, and applying a single tread across the whole vehicle is usually a compromise rather than an optimisation.

Fifth, write the pairing and acceptance rules into the contract annex. State the permissible ranges for diameter consistency, runout and hardness together with the verification method, so that the assembly side and the purchasing side share the same basis.

Sixth, let the maintenance strategy follow the configuration. Re-measure diameter quarterly on differential drive, watch the scrub band and interface fasteners on steer drive, and watch roller condition and floor maintenance on a Mecanum layout. Do not cover all three configurations with one maintenance sheet.

Seventh, include the supplier's process control capability in the evaluation. The long-term consistency of a wheel comes from process management and batch data, not from the test result of a single sample, and this should be confirmed clearly at the first cooperation.

Seen in this light, wheel selection for AGV projects is not a matter of picking a well-known part number. It is the discipline of establishing the load conditions first, letting the drive configuration determine which indicators matter, and then writing those indicators into a specification that can be measured, paired and traced. That sequence, developed from HANKE's engineering practice on AGV and production-line wheel applications, is what turns a configuration decision into a wheel specification that holds up over years of operation.