Shunting Principles

Shunting Principles

Moving rail cars requires engineering solutions that ensure that the effort being generated is not wasted. Additionally, moving to the railcars location quickly and easily, is important. Uniloks are engineered to optimize weight, traction and torque to deliver maximum shunting force, where it's needed. This' combined with manoeuvrability and versatility' makes it one of the most effective shunters on the market today.

Drawbar Pull

The pulling force at the locomotive couplers, or drawbar pull, is really what is required from shunters. To get the appropriate drawbar pull to start a train, a machine must transfer the engines power to the coupler through the wheels, onto the track without spinning. Uniloks optimize this process every step of the way.

Power and Design

Traditional locomotives operate on the principle of horsepower and weight, performing effectively at speeds. Yet when shunting, where speeds are low, horsepower is relatively unimportant but weight and traction are. This makes a loco's performance very inefficient and expensive for the task.
Road / rail shunters are designed specifically to perform the shunting task, at the same time providing maneuverability, versatility and cost effectiveness.

Torque and Horsepower Requirements

The torque required to give tractive effort at the wheel results from the engine torque amplified through the transmission. Under normal shunting speeds, only 40 to 80 horsepower of engine torque is required to move up to 1,000 tonnes. High horsepower is only required for higher speeds, not normally required when shunting. Uniloks are fitted with an automatic hydrostatic speed controlled transmission as standard, giving the highest tractive effort per unit horsepower in the business. The electronic processor automatically selects the best transmission gear based on the power required to move the load.

Tractive Effort

Tractive effort is the force exerted by a locomotive on the track for the movement of a train.
Tractive effort = Machines Operating Weight X Friction at the Wheel
Uniloks typically weigh 7 - 8 tonnes in the case of the D -125 , E -125 and E-125s and up to 23 tonnes for the A-55s, C-55s and E-55s (depending on model).
To allow the Unilok to perform task as machines many times their weight and cost, the D-125 , E-125 and E-125s models use the patented weight equalizing coupler to increase the machines "operating" weight. The coupler is designed, not just to borrow weight, but to effectively transfer the weight to all of its wheels.
If no wheel slip occurs, tractive effort and draw bar pull are equal.


Slippage on the rail can be the downfall of all shunting efforts. As starting resistance is 2.5 times that of rolling resistance friction levels are critical to a shunter's effectiveness. Slippage is due to a low friction or wheel spin.
Steel wheels on steel rails have very low adhesion or friction, typically only 0.2 to 0.3. Rubber tyres have a higher adhesion on steel, but this becomes very low under typical operating conditions when the rails are wet, iced or have leaves on them. Uniloks, incorporate high adhesion polyurethane tyred steel rail wheels, with a rail to wheel adhesion level of 0.8 on dry track and 0.5 on wet track.
In addition to higher rail to tyre friction, all Uniloks are fitted with a limited slip differential to help prevent slippage. The machine automatically reduces the speed to bring the slippage under control. On the Tractronic Shunters wheel slippage is eliminated by the hydrostatic drive and tracktronic control.