AMR Tugger Tow Capacity: Drawbar Pull, Cart Resistance, and Starting Loads

When an AMR tugger is specified for a towing application, the conversation usually centers on one number: how many kilograms can it tow? This number, however, conflates two different physical quantities and obscures the real question, which is whether the robot's drawbar pull can overcome the actual resistance of your carts under your actual floor conditions.

A tugger that is rated to pull 1,000 kg on a smooth, level floor may fail to start a 500 kg cart train on a 2-degree incline with misaligned casters. The rated tow mass tells you what the manufacturer tested under specific conditions. It does not tell you what will happen in your facility.

This article separates tow mass from drawbar pull, explains the factors that determine real cart resistance, and provides a framework for measuring your actual towing demand and matching it to manufacturer ratings.

For the earlier transport-architecture decision, compare underride, tugger and lift-top AMRs.

Tow Mass and Drawbar Pull Are Different Quantities

Tow mass is the combined mass of the permitted carts and loads under the manufacturer's stated towing conditions. It is expressed in kilograms or, in some catalogues, pounds of load. Distinguish this mass rating from pulling force.

Drawbar pull is the actual pulling force the tugger can exert at its coupling point. It is expressed in Newtons (N) or pounds-force (lbf). Drawbar pull is what overcomes rolling resistance, grade resistance, and acceleration forces.

These are not interchangeable. A tugger with a high tow mass rating may have a relatively low drawbar pull if the rating was established under favorable conditions: smooth floor, aligned casters, level surface, steady speed. Under less favorable conditions, the same tugger may not generate enough pull to move a much lighter load.

Manned tow tractors illustrate the distinction. Crown's TR 4500 page lists a 10,000 lb load capacity and drawbar pull of 250 lb normal / 700 lb maximum. Interpreting those drawbar figures as pounds-force gives approximately 1,112 N and 3,114 N. Linde's P20 page lists a 2,000 kg load capacity and rated drawbar pull of 400 N. Keep each value's label and test context: Crown's maximum figure is not directly comparable with Linde's rated figure, and these examples do not establish an AMR's towing capability.

These parameters describe manned industrial equipment and are provided here as terminology examples. They must not be transferred to AMR tuggers. AMR manufacturers publish their own ratings under their own test conditions, and the relationship between tow mass and drawbar pull for an AMR may differ significantly from manned equipment.

Measure the Cart Before Sizing the Tugger

The resistance a cart presents to the tugger depends on several factors that are specific to your carts and your facility:

Caster type. Swivel orientation and the effort needed to rotate the caster can add to starting and turning demand. Once aligned and rolling, resistance depends on the wheel, bearing, load, and surface; an all-swivel arrangement is not automatically harder to tow in every condition.

Bearing type. Ball bearings have lower rolling resistance than plain bearings. Carts with worn or unlubricated bearings require more force to start and maintain motion.

Wheel diameter. Larger wheels have lower rolling resistance than smaller wheels for the same load, because they encounter less resistance from floor irregularities.

Caster alignment. Misaligned swivel casters can substantially increase starting or turning demand. Include representative initial caster orientations in the force measurements. Do not treat this as the most common AMR tugger failure without a defined failure dataset.

Load weight. Rolling resistance is proportional to the normal force on the wheels. Heavier loads require more pull force.

Floor condition. Joints, debris, surface deformation, gradients, and the interaction with the wheel material can change towing demand. Compare the actual routes and coatings rather than assuming every coating increases resistance or that one surface always gives the lowest resistance.

Separate Starting, Steady Motion, and Grade Demands

The towing force required varies across three operating conditions:

Starting on a level floor. Measure the cart train's breakaway demand under defined load and caster conditions. It includes rolling-start, bearing, wheel-deformation, and caster-alignment effects; it is not generally the sliding-friction expression mu_static × N. A preliminary dynamic estimate may add m × a to a quasi-static breakaway measurement, but do not add acceleration twice if it is already included in the measured pull.

Steady motion on a level floor. Use the measured steady pull at the required load and speed. The approximation F_steady ≈ c_r × m × g may be used only where c_r is defined as an effective dimensionless rolling-resistance coefficient for those conditions; it is not a universal material constant.

Motion on a grade. For a simplified uphill force balance, F_required ≈ F_rolling(grade conditions) + m × g × sin(theta) + m × a. A model using c_r × m × g × cos(theta) for rolling resistance requires a coefficient supported for the cart and surface. Confirm signs, slope direction, speed, and whether measured data already include the grade and acceleration terms.

Starting uphill can be a demanding case, but turning casters, crossing joints, or other route events may create a higher peak. Measure the relevant combinations, compare peak duration and repeated demand with the manufacturer's ratings, and evaluate downhill control and braking separately.

Define the resistance terms and their units explicitly. An effective coefficient inferred from measurements is valid only within the tested load, wheel, surface, and speed conditions. A generic static-friction coefficient is not a substitute for measuring a wheeled cart train.

Document the route using AMR floor-readiness checks before treating a measured pull as representative of the whole facility.

Match the Duty Profile to the Manufacturer Rating

AMR tugger manufacturers publish ratings that describe what the robot can do under specific conditions. To determine whether a tugger meets your requirements, you need to compare your measured demand against the manufacturer's rated capacity.

The following table connects the operating conditions to the manufacturer ratings you need to confirm:

Ask the AMR manufacturer these questions before accepting a tow mass rating:

1. What is the continuous drawbar pull rating, and under what test conditions (floor surface, speed, temperature)?
2. What is the short-term maximum drawbar pull, and how long can it be sustained?
3. What is the thermal protection limit? How long can the robot pull at maximum force before it derates?
4. What is the grade capability rating? What incline can the robot start and maintain?
5. What speed is maintained under rated load conditions?

Keep the distinction between AMR payload ratings and real loads separate from the towing-force calculation.

If the manufacturer cannot answer these questions with specific test conditions, the tow mass rating is not sufficient for engineering evaluation.

Keep Coupling and Braking Limits Separate

A tugger that can pull a cart train may not be able to stop it safely. The coupling mechanism has its own load rating, and the braking system has its own deceleration capability. These are separate from the drawbar pull rating.

When a tugger decelerates a cart train, the carts push forward against the coupling. The coupling must withstand this compressive load without releasing or failing. The robot's braking system must decelerate the combined mass of the robot and the cart train within a safe distance.

Coupling and braking evaluation is outside the scope of this article, which focuses on drawbar pull and cart resistance. But these limits must be evaluated separately before deployment. A robot that can pull the load but cannot stop it safely is not a viable solution.

Prepare a Tow-Demand Evidence Pack

The output of this process is an evidence pack that documents your measured towing demand and the questions you need the manufacturer to answer:

Force measurements should be conducted by qualified personnel following a safety plan. A cart train that breaks free during a pull test can cause injury or damage.

Before selecting a tugger by kilograms alone, bring together your loaded-cart specifications, route conditions, measured starting and steady pull where available, and towing duty profile. AIsirRobot can help organize these inputs for comparison with candidate manufacturers' rating conditions. Traction, coupling, braking, and any on-site pull tests must be evaluated by the responsible manufacturer, integrator, and qualified site team.

Compare Tugger Ratings with Your Tow Demand