In screw jacks, where the input shaft is at a right angle to the lifting screw, worm drives (also known as worm gear sets) are right-angled drives. Bevel gears and hypoid gears are other varieties of right angle drives. Worm drives are ideal for use in systems utilising, for example, lifting equipment where a high gear ratio suggests it can be driven by a small motor because they meet the requirements of many systems and offer a compact way to decrease speed while increasing torque.
Worm gear, also known as a worm screw or simply a worm, and a worm wheel make up a worm drive. The worm wheel resembles a spur gear in appearance. The worm gear typically takes the shape of a screw with a 20° flank angle. Depending on the reduction ratio of the gear set, the worm gear screw can either have a single start or multiple starts.
The worm wheel has many teeth on a large diameter and a relatively small number of threads on a small diameter. A wide range of gear ratios, typically between 4:1 and 300:1, are available with this combination.
A worm drive's low efficiency makes it ideal for applications that only need it occasionally rather than constantly. The sliding contact between the teeth is what causes the worm drive to function inefficiently. To dissipate the heat produced and lower the rate of wear, the appropriate and sufficient amount of lubrication must be applied.
The worm wheel is frequently made from bronze or cast iron, and the worm gear is made from case-hardened steel with a ground finish for long life. Modern non-metallic materials are used in light duty applications, along with other material combinations when appropriate.
Worm Gear Construction
It is frequently necessary for a screw system (like the one in a screw jack) to prevent “back-driving” when the holding force is released and an axial load is applied. Since the shallower helix angle increases friction between the threads and is typically sufficient to prevent slippage, a single start thread is frequently used in these circumstances. A self-locking system is one that does just that.
This is predicated on a statically loaded system with minimal to no vibration, as this could lead to the combination becoming looser and the friction angle being overcome. It is recommended to use a locking mechanism or brake in systems that are subject to vibration to stop back-drive.
A multistart thread may be used if a system does not require self-locking but does require a faster rate of translation. This suggests that the screw shaft is
with multiple thread forms.
A single helical thread that wraps around a screw body is known as a single start thread. The form has advanced axially by the pitch of one thread for every 360° rotation of the screw. This is equally valuable to the pitch. Lead and pitch are equal for a single start thread.
Two threads form when a thread double starts. The combined pitch of two threads allows the forms to advance axially during a 360° rotation. Pitch is doubled by lead.
Three threads begin at the same time. The combined pitch of the three threads allows the forms to advance axially during a 360° rotation. Pitch is 3 times the lead.
Summary
A system with multiple start threads is less likely to be self-locking because the steeper helix angle of the threads results in less friction between them. As a result, a steeper helix enables faster translation along the threads; for example, a multi-start thread allows an object to be tightened in less rotations than a single-start thread does.
The linear output speed range we can provide is increased by incorporating different multi-start threaded worm shafts into Kelston screw jacks.
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