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With over 40 years of experience in the field of worm gearing, GIRARD TRANSMISSIONS has produced a range of worm gearboxes for applications where high accuracy, minimum backlash, heavy duty cycles (many thousand cycles/hour) are required such as packaging and handling machines, press feeders any kind of servo-systems.
In addition to the gearbox range, standard worm gear sets or customer specific
solutions can be provided. Typical fields of application; Rotary tables of
machining centres, Robots, Milling heads...
The general design of the gearbox is the same as conventional units. However the
worm gear set is of very special design and manufacture. The opposite flanks of
the worm threads are ground with slightly different pitches so that a variation
of the thread thickness is obtained when shifting the worm axially (see sketch
below).
The wheel is hobbed using a special process, so that the opposite flanks of a
same teeth also have different pitches, in order to be perfectly matched to the
corresponding flanks of the worm threads. This is the only way to ensure perfect
homokinetic input and output speeds, lowest transmission errors, and a very low
backlash value (always
less than 1 arc-min) that other systems (such as reduction of centre distance)
cannot reach.
Worm wheel sets with adjustable backlash
Applications: Worm wheel sets with adjustable backlash are recommended where accurate and smooth transmission of the movement (eg. printing machines) or accurate positioning (eg. Indexing) are required.
Selection: Look up tables are available for selection subject to accurate
assembly and correct lubrication. Special ratios on request.
Assembly: Housing and bearing assembly should allow axial shifting of worm to
achieve backlash adjustment. Bearings must provide maximum axial stiffness on
the worm as well as on the wheel (use tapered roller bearings or thrust ball
bearings or combined needle roller bearing. It is recommended, whenever possible
to use the standard adjustment device (available from Girard) which is supplied
assembled and preset ready for use.
It is important to assemble the worm and the wheel in accordance with the instructions (refer to handbook). A mark on the worm overall diameter indicates the position where the backlash is theoretically zero with a new set. The wheel must be accurately cantered in respect to the worm. Insert shims between wheel bearings and their abutment to obtain zero axial play without seizure. Then coat the worm threads with blue and assemble it. Now turn the worm shaft by hand in both directions while exerting a light braking effect on the wheel and observe the contact pattern.
Centre the wheel by transferring the shims from one side to another until a
symmetrical pattern in obtained on both flanks. Normally the contact should be
slightly shifted to the leaving side on both flank as shown below.
Backlash measurement
Backlash measurement should be carried out as follows :
firmly stall the worm shaft
fix a rod on the output shaft
at distance R from the output shaft axis place a dial Indicator on the housing the bock of which should be in contact with the rod
apply a light torque in both directions to the wheel shaft to ensure contact between worm threads and wheel teeth
the backlash (minutes of arc) is equal to the total indicator reading divided by R and multiplied by 3438
the instruction service leaflet supplied with each gearbox or gear set states the formula allowing calculation of the backlash modification as an amount of the shift in the worm shaft
Extract: BR series handbook Dec 1998 by Girard Transmissions.