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Caster Train Towing Force

Work out the drawbar pull needed to tow a train of carts, where every swivel caster adds swivel drag and every join adds a coupler loss.

Towing a train costs more than towing the sum of its weight, because every swivel caster adds drag and every coupler adds a loss. Enter the cart count, the weight per cart, the rolling resistance, the swivel and coupler factors, and the gradient, and the calculator returns the drawbar pull the tug or the operator has to supply.

The calculation is steady state: it does not include the peak when the train starts, the snatch when a coupler takes up its slack, or a brake left on or a caster jammed somewhere in the train. The swivel and coupler factors are screening allowances the user supplies, because they vary so much between fleets.

Inputs

Train and cart

Range: 1–30

Range: 20–6,000 lbf

Range: 2–8

Resistance

Range: 0.005–0.05

About 0.01 on a smooth sealed floor with hard wheels, 0.02 typical on concrete, 0.03 or more on carpet or a rough floor.

Range: 0.2–3

How much harder a swivel caster is to turn than a fixed wheel. 1 is normal. Heavy-duty swivel casters and sealed precision bearings drag more, and dirty bearings drag far more.

Range: 0–0.2

The extra drag each coupler adds as the train articulates. A rigid train has almost none; a loose coupler adds a lot.

Range: -10–10 %

Range: 20–2,000 lbf

The drawbar pull the tugger or the operator can deliver.

Result

Pull per cart in the train (lbf)
25.7
Total drawbar pull needed (lbf)
154
Share of the available pull used (%)
103
Verdict
The train is too heavy to pull within the available effort. Shorten it, fit lower-rolling-resistance casters, or use a powered tugger with more drawbar pull.
Train length note
The train length is short enough that the pull grows roughly in proportion to the weight. Beyond about ten carts the swivel drag and the articulation make the pull grow faster than the weight.

What to check

  • Count every cart in the train, including the one being towed empty.
  • Use the swivel and coupler factors your own fleet records support, not a default.
  • Check the steepest point on the route, because gradient adds to the steady pull.

Related calculators

A steady-state pull calculation. The start-up transient, the peak when a coupler straightens, and the snatch loads when slack is taken up are not modelled. Every cart is assumed to have the same weight and casters and to roll freely, so a brake left on or a jammed caster changes the whole train. Aerodynamic drag is ignored, and the swivel and coupler factors are screening allowances rather than measured values.