Twisted Belt

Providing power to a threshing machine is a mix of science and serendipity.
The threshing machine became reality largely due to the advent of the steam
traction engine. These behemoths were difficult to maneuver, fire belching
boilers that required a crew to operate. Thus most other farming tasks
continued to be done with horses. This included delivery of the unthreshed
crop to the thresher as well as hauling away the cleaned grain.

The threshing machine itself was a linear arrangement whereby the raw crop
was put into the feeder with the cleaned grain emerging somewhere amidships
and the straw and other effluent expelled from the rear. The threshing
machine, usually wooden bodied in the early versions, processed highly
flammable product. The steam boiler was a noisy, smoke and fire emitting
dragon that frightened the horses and threatened the product. A long belt
was thus used to allow removing the power unit from the immediate vicinity.
It was also difficult to exactly align the driveR and driveN shafts. The
greater the separation between dR and dN, the less the impact of
non-parallel shafts.

The 180 degree twist in the belt has effect in three broad categories;
tracking, stability and efficient power transmission.

1) Tracking. A flat belt generally tracks to the high (or tight) side. It
will climb on a drive that is not perfectly aligned. Twisting the belt
averages the edge tension to reduce that effect. This is not a cure-all.
The system must still be carefully aligned. The twist will make the belt
path stable but too much alignment error will cause the belt to run inboard
on one pulley and outboard on the other. Obviously pulley width is the
limiting factor. Crowned pulleys can also help.

2) Drive stability. This is a catch-all category the includes several
topics such as belt droop, belt "flop", belt operating envelope; all of
which greatly impact operational safety.

The longer the belt span, the greater the effect of tight side versus slack
side tension. On a threshing machine drive the slack side of an untwisted
belt can actually drag on the ground. The flywheel of the power unit
provides relatively stable drive input. The threshing unit however will
surge greatly depending on the variations in product throughput. Thus the
slack side droop can vary wildly. If this belt flop remained in the strict
vertical plane it might be acceptable. However in practicality, a wildly
gyrating belt can throw itself from the drive resulting in a long, 100+
pound unguided projectile. Even if the belt stays on the drive, the flop
and slap will scare the horses who must stand patiently as the rack they are
drawing is emptied. Excessive flop will also tend to relocate over time the
dR and/or dN units thus further impacting system alignment and tension.

A 180 degree twist toward the center of the belt span serves to limit the
drive envelope variations, a boon to machine, man and beast. Also, a minor
but important feature of twisting the belt drive is to limit trash enclusion
in the drive. Trash is either rejected outright or gets cleared as the belt
surfaces invert.

3) Efficient power transmission. Perhaps the single most critical element
in a flat belt drive system is pulley wrap. The greater the surface
wrapped, the greater is the drive's power capability. A twist in the belt
both maximizes and stabilizes pulley wrap. With equal diameter pulleys and
a twist in the belt one can get better than 180 degree wrap (increased drive
rating for the same belt dollar).

Although not such a fire hazard, the less massive firing engine tractors are
not as inherently stable a power source as is the steam engine. This
increases all negative elements of the drive and makes it even more
desirable to have a twist in the drive belt.