By Michael Adler
A major problem in geared precision instruments is that of reducing
backlash. This occurs in a train of gears where the final drive will lag behind the
input, and happens each time the drive is reversed. In fact it is a problem only
where violently oscillating or interrupted functions arise that backlash merits further consideration. This occurs in designing and writing machines, and also in the Analogue Differential
Analyser used for solving calculus equations, where a high degree of precision is required.
It is possible to reduce this effect by using gears with carefully cut
teeth while at the same time allowing sufficient clearance for free running. High gear reduction ratios also help by reducing the effect of
A mechanism known as a frontlash unit has been developed. This is inserted before or after the drive requiring correction, and if properly adjusted,
can cancel out backlash entirely. Essentially it is a unit introducing an adjustable amount of 'negative backlash' or 'frontlash', thus
compensating for the backlash inherent in the drive. It works on the following way: It normally provides a rigid connection between its input and output shafts, but when the
direction of rotation is reversed, the outgoing shaft is caused to rotate faster than the input for an adjustable amount, during
which the lag due to backlash is recovered. There is zero backlash therefore in the
system, except for the short period just after reversal occurs so that the resulting error is cancelled out.
The main parts of the unit are as follows:
A pin is fixed to the input shaft and projects at a right angle from it.
A drum is mounted loosely on the input shaft, and has two projecting
pegs, whose angle to each other can be adjusted between 0 and 360 degrees. A friction band is applied to the rim of the drum. When the pin is in contact with
either of the pegs, a direct drive is established between the input shaft and the drum and the unit rotates as a whole, slipping
the friction band. When the direction of input rotation is reversed, the pin moves
between one peg and the other by the determined amount, the drum being held back by the friction band.
Fixed tightly to the input shaft is a planetary gear train. This engages on the input side with a gear fixed to the drum, and on the output side to an
internal gear connected with the output shaft.
In the direct drive, with the pin and one peg in contact with one
another, none of the gears in the planetary train rotates with respect to any other, though all have a common motion about the
axis. During a reversal, whilst the pin moves from one peg to the other, the drum
being held stationary by the friction band, the planetary gear train causes the driven shaft to rotate faster than the input. By adjusting the angular separation between the pegs, the amount of this movement can be
controlled, thus canceling out any backlash in the system requiring correction. The friction band is not a serious increase in the torque on the input shaft.
The adjustment is carried out as follows:
The drive is marked at each end, and the input is rotated several times,
and the final position of the output shaft is noted with respect to the input. Then
the input shaft is rotated in the opposite direction until it attains its original position, and this is repeated several times,
each time adjusting the distance between the pegs until the output shaft takes up its original position This indicates that the backlash has been successfully eliminated.
The mechanism can be seen in the illustration. The input shaft runs right through from front to back. A crank is fixed on the shaft, and this can be moved between two threaded pins.
One of these pins is fixed to a large contrate and this is fixed to a drum, which is a wheel flange. A second contrate carrying a second pin meshes tooth to tooth with the first, and is
pressed against it by a compression spring between crank and contrate.
Fixed behind the drum is a bush wheel, and a 1/2" pinion is fixed to
its boss using a socket coupling. First contrate, drum, bush wheel, socket coupling
and pinion this move as one unit, free on the input shaft.
A planetary gear train is fixed on the input rod, one end of which meshes
with the drum, and the other with an internal gear ring connected to the output. The
cage is built up from a bush wheel and a wheel disc bolted together using
two 3/4" bolts. Two 1/2" pinions are fixed on 1 1/2" rods between the
wheels. Two further 1/2" pinions are fixed to these rods, and mesh with the
previously mentioned 1/2" pinion held in the drum socket coupling. The output of
the cage meshes with an MW gear ring (part 180A) which has 57 internal teeth. This
ring is bolted to a face plate, free on the rod. A 3/4" pinion lies behind the
face plate, and is fixed to it by a second socket coupling. This pinion meshes with
an offset output 50 tooth gear.
A friction band bears against the rim of the drum. It is an evenly bent 5 1/2" narrow strip, which is fixed by a threaded coupling to
the side of the mechanism. The band tension can be varied by adjusting a screw at its
other end. The screw is a 3/8" bolt which is passed through the end hole of the
strip, and enters a threaded boss which is fixed to the frame using a fishplate.
Action. The amount of front
lash can be selected by varying the amount of displacement of the pins on the contrates. This
is done by sliding the front contrate forward along the input rod against the
compression spring. The input crank can thus move between the pins by a varying
amount. When the input gear is turned, the crank moves between the pins. The drum is prevented from turning because of the friction brake. The planetary gear train fixed on the input shaft continues to rotate, and its gears run
around the stationary output pinion of the drum. The internal gear ring is caused to
rotate . As the step up drive between planetaries and gear ring is 3:1, the output
rotates faster than the input, until the input crank contacts a pin. The whole unit then rotates, with no planetary gear motion. When the mechanism is reversed, the crank moves between pins once more, and the output
rotates faster in the opposite direction.
This unit has been designed as a demonstration of the action of a
frontlash mechanism. The planetary gears could be replaced by a differential unit,
whose inputs can be varied according to requirements, and whose output is the cage of the differential.