Worm drives (or worm gear sets) are right angled drives and are used in screw jacks where the input shaft is at right angles to the lifting screw. Other forms of right angle drives are bevel gears, and hypoid gears. Worm drives satisfy the requirements of many systems and provide a compact means of decreasing speed whilst increasing torque and are therefore ideal for use in systems utilising e.g. lifting equipment where a high gear ratio implies it can be driven by a small motor.
A worm drive consist of a worm wheel and worm gear also known as worm screw or just worm. The worm wheel is similar in appearance to a spur gear the worm gear is in the form of a screw generally with a flank angle of 20°. The worm gear screw can be single start or have multiple starts depending on the reduction ratio of the gear set. The worm has a relatively small number of threads on a small diameter and the worm wheel a large number of teeth on a large diameter. This combination offers a wide range of gear ratios typically from 4:1 to 300:1.
The low efficiency of a worm drive lends itself to applications that require intermittent rather than continuous use. The worm drive inefficiency originates from the sliding contact between the teeth. Appropriate and adequate lubrication must be applied to dissipate the heat generated and reduce the wear rate. For long life the worm gear it made from a case hardened steel with a ground finish and the worm wheel is often made from bronze or cast iron. Other material combinations are used where appropriate and in light duty applications modern non-metallic materials are deployed.
Often a screw system (such as that found in a screw jack) is required not to 'back-drive' when the holding force is removed and an axial load is applied. A single start thread is commonly used in these situations as the shallower helix angle causes greater friction between threads and is usually sufficient to prevent slippage. Such a system is said to be self-locking. This assumes a statically loaded system with little or no vibration as this may cause the friction angle to be overcome and the combination to untighten. In systems that are subject to vibration a locking mechanism or brake is advised to prevent back-drive.
If self-locking is not a requirement of a system but a greater speed of translation is then a multi start thread may be used. This implies that multiple thread forms are created on the screw shaft.
Single Start Thread: A single helical thread formed around a screw body. For each 360° revolution of the screw, the form has advanced axially by the pitch of one thread. This has the same value as the pitch. In the case of a single start thread, lead and pitch are equal.
Double Start Thread: Two thread forms. During 360° revolution the forms advance axially by the combined pitch of two threads. Lead is 2x the pitch.
Triple Start Thread: Three thread forms. During 360° revolution the forms advance axially by the combined pitch of three threads. Lead is 3x the pitch.
A multi start thread has a steeper helix angle which results in less friction between the threads and therefore such a system is less likely to be self-locking. It follows that a steeper helix allows for faster translation along the threads i.e. an item utilising a multi start thread can be tightened in fewer rotations than one using a single start thread.
Incorporating various multi start threaded worm shafts in to Kelston screw jacks increases the linear output speed range we can offer.