Product category:
Drives, motors and power transmission, couplings, clutches
News Release from: Holroyd | Subject: Worm gear systems
Edited by the Manufacturingtalk Editorial
Team on 31 July 2003
How to remove backlash from worm gear
systems
Backlash is the enemy of precision in high accuracy worm gearing systems, but there is a solution, says Steve Whitehead, director of precision gear sales for Holroyd.
When telescopes scour the heavens for signs of the origin of the universe, or dicing machines cut the latest in silicon chip wafers, there is a more than even chance that both will be employing the same type of gearing system, in the shape of precision worm gear sets Increasingly worm gears are first choice, not only in the above applications, but also for others such as machine tool tables, cutter drives on milling machines, robotic arms and roller drives on printing machines
This article was originally published on Manufacturingtalk on 14 Aug 2001 at 8.00am (UK)
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In all of these applications, extreme accuracy requirements mean that it is imperative backlash in the gearing system be kept to an absolute minimum.
There are a number of ways of achieving this, including: * Varying the gear centre distance.
* Splitting the wormshaft.
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* The use of a dual lead, which may also be termed a duplex type gear.
The question is: which one is the most effective? Varying the gear centre distance - this method achieves a limited amount of success but it does not give a true meshing condition on the wheel teeth.
In addition, any deviation from the true gear centre distance may give irregular rotation of the gears, which cannot be tolerated on precision drives.
Splitting the wormshaft - with this arrangement the worm is produced as a two-piece item, the design being split at the gearing centreline.
One worm may be integral with the shaft, the other keyed or splined to the shaft but axially adjustable, the position being fixed by a spacer positioned at the centreline between the two components.
The theory behind this arrangement is that while one worm drives in one direction, the other is adjusted to remove backlash and to drive in the opposite direction, if required.
There are a number of disadvantages with this method.
Firstly, the gap required by the spacer can cause an interruption of the line of contact.
Secondly, when adjustment is called for the wormline has to be dismantled partially in order to remove the spacer, an operation that is very time consuming.
Dual lead wormgear - the dual lead wormgear does not suffer from any of the above disadvantages.
It provides a kinematically correct gear, which can run in either direction of rotation and on which the backlash is infinitely adjustable from an acceptable maximum to zero.
In addition, the dual lead wormshaft is a one- piece unit that is less expensive than and also more accurate than split worms, including Class 1 types.
As the name suggests, dual lead wormgears are constructed with two leads.
One flank of the worm thread and its mating sides of the wheel teeth are manufactured with one lead, and the other flank and its mating wheel teeth to a slightly different lead.
As far as the worm is concerned, this has the effect of producing a worm thread, which increases in thickness from one end of the worm to the other.
The advantage of this design is that, if the worm is moved in an axial direction when in mesh with the wormwheel, backlash between the pair can be adjusted to the required amount.
In practice the gears are manufactured so that the initial backlash is achieved at a certain distance relative to a datum plane on the worm.
The worm can then be adjusted from this point as and when necessary during the life of the gears.
The amount of adjustment provided for in the gear varies according to requirements but can be between 120 microns to 500 microns for each 25.4 mm of axial movement, the majority being standardized at 325 microns (.013in) per inch of adjustment.
When fixing the centre distance and gear data, the load capacity of the wormgear set should be such as to provide an adequate gear life with a Iow rate of wear with only very occasional need for backlash adjustment.
It is usual not to have to make this adjustment more than two or three times throughout the life of the wormgear set.
The accuracy of a set of gears takes account of the tolerances of concentricity, profile, adjustment, accumulative pitch errors, and centre distance.
When assessing a requirement for a dual lead worm gear it is necessary to confirm that the centre distance of the supporting case can be held within plus 0.05 (0.002in) minus 0.00mm.
There are various national standards covering the accuracy and classification of wormgear sets which stipulate maximum allowable errors of adjacent and cumulative pitch and profile.
Holroyd have the manufacturing capability to achieve closer tolerances than are stated in the majority of these standards with the necessary equipment for their checking and verification.
In addition, Holroyd have unique equipment for carrying out single flank or transmission testing.
This comprises a continuous measurement of the angular position of the driven wormwheel relative to where it should be with a perfect gear set as the driving worm rotates at a uniform rate.
The test measures, and records directly, pitch and eccentricity errors, and also the profile errors of the active part of the profile.
Holroyd manufactures dual lead wormgears in nominal ratios between 5 and 280:1, dependent upon centre distance and pitch.
The minimum pitch practicable is usually 5mm (0.20in) and minimum centre distance 75mm (3in).
It is usual not to adopt an actual ratio in cases where the number of teeth in the wormwheel is less than 30, as this keeps transmission errors to a minimum.
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