Tracklayer Development

The Lombard log hauler was the forerunner of the Holt and Best crawler tractors, which like the Lombards, were steered by turning guiding skis or wheels. A common early term for track-type tractors was “Tracklayer,” which was based on an analogy to railroad tracks. Tracklayers laid their track ahead and then picked it up from behind as they moved.

Early in the development of these steerable track machines, it was not obvious that there was a better way to achieve steering rather than steerable skis, or wheels in the case of the Holt and Best tractors. The better way was to selectively de-power and apply braking to either track, which caused the tractor to turn. Such a method did not require steerable skis or wheels to influence the turn. This improved steering is what we have today in modern crawler tractors.

I like to study track system details because many different ideas have been tried out since the beginning. Modern tracks are made from link chains with plates bolted on to them. Another earlier version was, as seen on the Lombard, which used plates directly hinged together without the chain. The plates then have their own features that engage drive sprockets.

Modern track undercarriages support most of the tractor weight on a bed of track rollers closely spaced along the length of the ground contacting track bed. It is important to place the weight uniformly on the track bed to provide the best weight distribution and traction.

The Lombard in the video does not have the series of fixed track rollers. Instead, it has a chain of “recirculating” rollers. The rollers act like placing pipes on the floor to roll a piece of heavy machinery during a move across a floor or up a ramp to a trailer, for instance. The pipes travel as they rotate just like the recirculating rollers in the roller chain seen in the video at 1:11, you can see the track system clearly showing the recirculating roller chain. The rollers in the top string of the chain are simply returning to the front.

The rollers in the bottom of the chain are transferring the weight of the machine from a fixed iron platen bearing on the top of the rollers, to the upper side of the track plates at the bottom of the rollers.

This ingenious roller chain system was later used on at least one Cletrac crawler tractor. The roller chain has also been called the “String of pearls.”

Here is a link to an informative discussion about crawler track history:

[www.practicalmachinist.com]

In the photo linked to the first post by Terry Harper, you can clearly see the recirculating roller chain. What appear to be round iron wheels at each end of the oval recirculating roller chain configuration are actually non-rotating, fixed, circular guideways. Bear in mind that this chain does not transmit power. It merely lays down a line of rollers to carry the weight of the tractor down to the track bed.

If you scroll down to the sixth post down from the top by Robert Grauman, there is a great photo and 3D cad illustration of the track parts of a Phoenix tracklayer, and the recirculating roller chains with their fixed guideways are clearly shown.