AGV/AMR Drive-Steering Unit Selection Guide: 9 Engineering Inputs

A drive-steering unit is one of the key motion components in an AGV or AMR chassis. It typically integrates the travel drive, reduction gearing, drive wheel and active steering mechanism into a compact module.

For an engineering project, a drive-steering unit should not be selected solely according to total vehicle weight or rated load capacity.

Two vehicles with the same total mass may require completely different drive configurations if they have different:

• travel speeds;

• acceleration requirements;

• gradients;

• wheel layouts;

• floor conditions;

• installation envelopes;

• duty cycles;

• control architectures.

An unsuitable drive-steering unit may lead to:

• insufficient tractive force during starting or climbing;

• wheel slip or abnormal tyre wear;

• slow steering response or increased positioning error;

• excessive motor, gearbox or wheel temperature;

• interference between the steering envelope and chassis structure;

• incompatibility with the vehicle control system;

• acceptable prototype performance but insufficient long-term reliability.

Before selecting a product series and configuration, the following nine engineering parameters should be reviewed.

1. Vehicle Mass and Load Distribution

Total vehicle mass is the starting point for product selection, but the actual load carried by each drive-steering unit cannot always be calculated simply by dividing the total mass by the number of wheels.

The following factors should also be considered:

• unladen vehicle mass;

• maximum payload;

• payload centre of gravity;

• number of drive-steering units and load-bearing wheels;

• position of each wheel unit within the chassis;

• dynamic loads generated during lifting, braking, cornering or crossing floor joints;

• local load transfer caused by uneven floors;

• suspension or floating mechanisms.

For example, when the payload centre of gravity is offset towards one side of the vehicle, the load carried by each wheel may differ significantly.

Spring-loaded or floating mounting systems can also change how the load is distributed between drive wheels and passive load-bearing wheels.

Information Required for Selection

• unladen vehicle mass;

• maximum fully loaded mass;

• payload dimensions and centre-of-gravity position;

• chassis and wheel-layout drawing;

• estimated static load at each support point;

          • details of impact loads, eccentric loading or lifting operations.

Engineering Recommendation

The rated load capacity should cover the maximum actual wheel load under the most demanding operating condition, not only the average static load.

The stiffness of the chassis, mounting plate and surrounding structure should also be reviewed.

2. Required Tractive Force and Wheel Output Torque

Load capacity determines whether the unit can support the vehicle. Tractive force determines whether the vehicle can start, accelerate, climb and continue moving under load.

The total tractive force can be estimated using the following simplified relationship:

Total tractive force ≈ acceleration resistance

                    + rolling resistance

                    + gradient resistance

                    + additional mechanical resistance

A simplified calculation is:

F ≈ m × a

 + Crr × m × g × cosθ

 + m × g × sinθ

 + Fother

Where:

• F = total required tractive force, in N;

• m = fully loaded vehicle mass, in kg;

• a = design acceleration, in m/s²;

• Crr = rolling resistance coefficient;

• g = gravitational acceleration;

• θ = maximum gradient angle;

• Fother = additional resistance from bearings, seals, mechanisms or auxiliary equipment.

Where multiple drive-steering units are installed, the tractive-force requirement must be distributed according to:

• the number of driven wheels;

• load distribution;

• chassis geometry;

• drive-control strategy.

The required wheel output torque can be estimated as:

Wheel output torque ≈ tractive force per wheel × wheel radius

Motor torque must then be evaluated together with:

• gear ratio;

• transmission efficiency;

• continuous and peak operating conditions;

• appropriate engineering safety margins.

Adhesion Must Also Be Checked

The maximum force that can be transmitted through the wheel is limited by wheel-to-floor adhesion:

Available adhesion force ≈ coefficient of adhesion × normal load on the drive wheel

Even when the motor can produce sufficient torque, the wheel may still slip if:

• the actual drive-wheel load is too low;

• the floor is dusty, oily or wet;

• acceleration is too aggressive;

• the gradient is too steep.

Information Required for Selection

• maximum fully loaded mass;

• maximum acceleration and deceleration;

• maximum gradient;

• number of driven wheels;

• actual load on each drive wheel;

• floor material and contamination conditions;

• additional resistance caused by towing, lifting or other mechanisms.

3. Travel Speed, Acceleration and Braking Requirements

The rated speed of the drive-steering unit should exceed the design travel speed of the vehicle. However, product selection should not be based only on maximum speed.

The following conditions should also be confirmed:

• normal operating speed;

• maximum travel speed;

• acceleration and deceleration time;

• frequency of starts and stops;

• low-speed positioning requirements;

• speed limits during cornering;

• emergency braking requirements;

• parking or holding-brake requirements.

Two vehicles with the same maximum speed may require different motor and gearbox configurations if one vehicle accelerates more rapidly or stops more frequently.

Frequent starting, braking and steering can increase the thermal load on:

• the motor;

• motor drive;

• gearbox;

• wheel tread;

• braking system.

Engineering Recommendation

The following should be checked together:

• motor rated speed;

• motor peak speed;

• continuous torque;

• peak torque;

• gear ratio;

• drive-wheel diameter;

• brake holding torque;

• drive overload capability;

• acceleration and deceleration profile.

A fixed universal speed range should not be used for all drive-steering applications. The final selection should be based on the actual product performance data and vehicle duty profile.

4. Gradient, Floor Conditions and Obstacle-Crossing Requirements

Floor conditions directly affect:

• tractive force;

• rolling resistance;

• tyre wear;

• vibration;

• operating noise;

• vehicle stability.

Relevant floor information includes:

• epoxy flooring;

• concrete flooring;

• tiles or other surfaces;

• floor flatness;

• joints, gaps and thresholds;

• gradients and ramps;

• dust, oil or moisture;

• lift gaps and embedded floor rails;

• indoor, semi-outdoor or outdoor operation.

A smooth, dry industrial floor creates different requirements from a rough floor with joints, contamination or gradients.

Information Required for Selection

• maximum gradient and ramp length;

• maximum threshold or floor-joint height;

• minimum available turning area;

• floor material;

• floor cleanliness;

• potential wheel-slip conditions;

• indoor or outdoor operating environment.

Engineering Recommendation

Rated tractive force measured under ideal floor conditions should not be treated as the only reference.

For gradients, rough floors or contaminated surfaces, rolling resistance, adhesion and safety margin should be reassessed.

5. Drive-Wheel Diameter, Material and Hardness

Wheel diameter, tread material and hardness affect:

• load capacity;

• rolling resistance;

• required drive torque;

• operating noise;

• vibration damping;

• floor protection;

• tread wear;

• the ability to cross joints and small obstacles.

A larger wheel diameter generally improves the ability to travel across floor joints and small obstacles. However, it also:

• increases installation height;

• increases the steering envelope;

• increases the required wheel output torque for the same tractive force;

• reduces wheel rotational speed at the same vehicle speed.

A softer tread can improve damping and reduce noise, but may produce:

• greater deformation;

• higher rolling resistance;

• increased heat generation under continuous load.

A harder tread may reduce rolling resistance and improve wear resistance, but generally places greater demands on floor flatness and chassis suspension.

Wheel selection should therefore consider:

• tread compound;

• Shore hardness;

• dynamic load capacity;

• rolling resistance;

• wear resistance;

• heat generation;

• floor compatibility.

Information Required for Selection

• maximum permitted wheel diameter;

• maximum available chassis height;

• floor type;

• operating noise requirements;

• non-marking requirements;

• exposure to oil, water or chemicals;

• expected operating hours per day;

• expected travel distance;

• tread-life requirements.

Engineering Recommendation

Wheel material and hardness should be selected according to the complete application.

It is not correct to assume that a harder wheel is always better, or that a larger diameter is always preferable.

6. Installation Envelope and Mechanical Interface

A drive-steering unit requires more than sufficient static installation space. Clearance must also be provided for:

• steering movement;

• cable movement;

• maintenance;

• component removal.

The following dimensions should be checked:

• total unit height;

• mounting flange or mounting-plate dimensions;

• mounting-hole pattern;

• steering-centre position;

• maximum rotating envelope;

• motor and gearbox dimensions;

• cable, connector and cable-chain clearance;

• ground clearance;

• installation and removal path;

• maintenance access;

• mounting-surface flatness;

• mounting-structure stiffness.

Common Integration Problems

A unit may appear to fit within the chassis in a static 3D model, but interference can occur when the steering axis rotates.

Possible issues include:

• the motor colliding with the chassis;

• cable strain during repeated steering;

• insufficient clearance around the gearbox;

• difficulty removing the unit for maintenance;

• inconsistent wheel heights;

• insufficient mounting-plate stiffness;

• deformation affecting steering accuracy.

Engineering Recommendation

The selection process should include:

• 2D drawing review;

• 3D model review;

• steering-envelope verification;

• cable-routing review;

• maintenance-access review.

The mechanical interface should be confirmed before the chassis design is finalised.

7. Motor, Gear Ratio, Brake and Encoder Configuration

Mechanical dimensions alone are not sufficient. The drive-steering unit must also be compatible with the electrical and control architecture of the vehicle.

The following should be confirmed:

• battery or DC bus voltage;

• motor type;

• motor rated power;

• continuous torque;

• peak torque;

• gear ratio;

• motor-drive brand and model;

• electromagnetic brake requirements;

• encoder type;

• encoder resolution;

• steering zero-position detection;

• cable and connector requirements;

• motor and drive protection rating.

Gear-Ratio Selection

A higher gear ratio can:

• increase output torque;

• reduce maximum travel speed;

• influence system response.

A lower gear ratio can:

• increase available speed;

• reduce wheel output torque;

• create insufficient torque during starting, climbing or heavy-load operation.

The gear ratio should therefore be selected according to:

• wheel diameter;

• required travel speed;

• required tractive force;

• motor speed;

• duty cycle;

• transmission efficiency.

Brake Function

A mechanical brake may be required for:

• holding the vehicle after power loss;

• parking on a gradient;

• emergency braking support;

• maintaining position under heavy load;

• safety retention during lifting operations.

The need for a brake should be determined through the vehicle risk assessment, control strategy and actual operating conditions.

8. Steering Accuracy, Control Method and Communication Interface

For AGVs and AMRs that require accurate path tracking, manoeuvring in confined spaces or coordinated multi-wheel steering, steering performance and control interfaces are critical.

The following parameters should be reviewed:

• maximum steering angle;

• continuous 360-degree steering requirement;

• steering speed;

• angular positioning accuracy;

• repeatability;

• mechanical backlash;

• steering zero-position detection;

• absolute steering-position feedback;

• synchronisation requirements for multiple units;

• command format from the navigation controller.

Possible communication and control interfaces include:

• RS-485;

• CANopen;

• EtherCAT;

• industrial Ethernet;

• pulse control;

• analogue control;

• digital I/O.

The actual available interface depends on the selected:

• motor;

• motor drive;

• controller;

• feedback system.

Multi-Unit Applications

Projects using multiple drive-steering units should also confirm:

• mechanical zero position of each unit;

• drive-speed distribution;

• steering-angle synchronisation;

• instantaneous centre-of-rotation calculation;

• communication delay;

• fault-state behaviour;

• consistency between encoder and navigation data.

If omnidirectional movement is required, the chassis kinematic architecture and number of steering units should be defined early in the vehicle design process.

9. Duty Cycle, Operating Environment and Validation Conditions

The same drive-steering unit may perform differently under occasional operation and continuous operation.

Duty cycle affects:

• motor temperature;

• gearbox temperature;

• wheel wear;

• lubricant performance;

• brake temperature;

• long-term reliability.

The following should be confirmed:

• operating hours per day;

• maximum continuous operating time;

• start-stop frequency;

• steering frequency;

• percentage of operation at full load;

• average speed;

• duration at maximum speed;

• ambient temperature;

• humidity;

• dust and moisture;

• cleanroom requirements;

• low-noise requirements;

• antistatic or conductive-wheel requirements;

• indoor or outdoor use.

Recommended Prototype Validation

Before series production, the vehicle should be tested under representative conditions, including:

• unloaded starting;

• fully loaded starting;

• maximum speed;

• maximum acceleration;

• maximum gradient;

• low-speed stability;

• steering accuracy;

• steering repeatability;

• braking and power-loss holding;

• motor and gearbox temperature rise;

• wheel slip;

• tread wear;

• continuous duty cycle;

• communication interruption;

• fault handling;

• endurance operation.

The final product selection should be confirmed through vehicle-level prototype testing, rather than theoretical calculations alone.

Six Common Drive-Steering Unit Selection Mistakes

1. Selecting Only According to Total Vehicle Weight

Vehicles with the same total mass may have different wheel loads, centres of gravity and dynamic conditions.

Wheel layout and maximum actual load at each support point must also be considered.

2. Treating Load Capacity as Drive Capability

Load capacity describes the mechanical load the unit can support.

Drive capability depends on:

• motor torque;

• gear ratio;

• wheel diameter;

• adhesion;

• actual normal load on the driven wheel.

3. Considering Only Maximum Travel Speed

Acceleration, frequent starts, gradients and steering frequency may have a greater influence on the motor and gearbox than maximum travel speed.

4. Ignoring the Steering Envelope

A unit may fit in its neutral position but interfere with:

• the chassis;

• battery;

• cables;

• lifting mechanism;

• surrounding structures

when it rotates.

5. Using the Same Wheel Material for Every Floor

Different loads, floors, speeds, noise limits and operating environments require different tread materials and hardness levels.

6. Moving Directly to Series Production Without Prototype Testing

Actual vehicle performance is influenced by:

• load distribution;

• control software;

• floor conditions;

• installation stiffness;

• real duty cycle.

Prototype validation should be completed before series production.

Recommended AGV/AMR Drive-Steering Unit Selection Process

Step 1: Submit the Application Data

Provide:

• vehicle mass;

• payload;

• speed;

• acceleration;

• gradient;

• installation space;

• floor conditions;

• duty cycle;

• electrical architecture.

Step 2: Identify Suitable Product Series

Candidate product series are selected according to:

• actual wheel load;

• required tractive force;

• wheel diameter;

• installation envelope;

• speed requirements.

Step 3: Confirm Mechanical and Electrical Interfaces

Review:

• 2D drawings;

• 3D models;

• mounting-hole pattern;

• motor;

• gear ratio;

• encoder;

• brake;

• communication interface.

Step 4: Complete Prototype Validation

Test the vehicle for:

• travel;

• steering;

• load handling;

• temperature rise;

• noise;

• wheel slip;

• endurance.

Step 5: Confirm the Series-Production Configuration

After testing, confirm:

• final product model;

• motor and gearbox configuration;

• connectors and cables;

• control parameters;

• drawing revision;

• delivery requirements.

Drive-Steering Unit Selection Checklist

Conclusion

Selecting an AGV or AMR drive-steering unit is a system-level engineering task.

The final configuration should satisfy:

• actual static and dynamic wheel load;

• starting, acceleration and gradient tractive force;

• travel-speed and braking requirements;

• floor and wheel-material conditions;

• installation envelope and chassis stiffness;

• motor, gear ratio, brake and encoder requirements;

• steering control and communication compatibility;

• duty cycle and environmental requirements;

• prototype validation and long-term reliability.

Providing complete application data at the beginning of a project can significantly reduce the risk of:

• mechanical interference;

• electrical incompatibility;

• insufficient tractive force;

• wheel slip;

• excessive temperature;

• repeated chassis modifications;

• delayed series production.

Need Support Selecting a Drive-Steering Unit?

Please provide your:

• vehicle mass;

• travel speed;

• available installation space;

• maximum gradient;

• floor conditions;

• duty cycle;

• control interface;

• estimated project quantity.

The JY Robot engineering team will help evaluate a suitable product series, mounting arrangement and configuration for your application.

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