Models by Alan Wenbourne

Cletrac Tractor

This is a model demonstrating the Cletrac Controlled Differential Steering system, or ‘a crawler with a steering wheel’.

I decided to re-visit this subject to see what alternatives there may be to the double differential system. The DD arrangement was used in my M4 HST Military Munitions Tractor (RMGM84 & CQ105) which proved exceedingly effective in controlling a relatively heavy track laying model vehicle. The model performed very well at speeds up to 6.5 ft/sec and demonstrated good steering control, in fact as good as a wheeled vehicle when under radio control.
However, the system required two variable speed drives to work effectively, and radio control via two separate electronic speed controllers. Not your typical Meccano components!

This made me wonder about modelling a simpler system such as the Cletrac drive, which uses a single differential with steering clutches. This is a controlled differential which overcomes the disadvantages of the skid-steer systems that waste energy. It does not directly brake either output shaft but proportions the drive to both, thereby facilitating a fixed radius of turn when one brake is fully applied dependent upon the ratio of the additional planetary gears. No power is wasted in applying the brake(s) so the system is regenerative.

The subject system was patented by Rollin H White, founder of the Cleveland Tractor Company. He pioneered the use of tracklaying machines before Caterpillar and others.

The tractor was steered by a steering wheel mounted on a vertical column, driving through a ‘V’ formation of gears to the screw jacks of the external contracting steering brakes. This ensured that both brakes could not be applied at the same time. The differential is a spur gear pinion design with an additional planetary set either side connected via their sun gears to the steering brakes.

The first tractor to be equipped with Cletrac controlled differential steering was the model ‘H’ in 1922. The controlled differential steering system was very successfully applied to all future Cleveland Tractors and was used in many military vehicles, APV’s and battle tanks internationally.

Look out for a full article describing this model on the South East London Meccano Club website soon...