Right Angle Speed Reducers
Right Angle Speed Reducers (commonly referred to as "Worm Gears") are simple in nature and able to achieve high ratios in a relatively small package, compared to a "helical" or "helical-bevel" gearbox. The operational concept is obtainable through the use of one of physic's "simple machines"... the Screw.
The following two images are courtesy of Wikepedia.org
|Worm gear (4-start worm, 50-tooth wheel, 1:12.5 gear ratio)||
By Rama - Own work,
The proper nomenclature of the two main components of the right angle speed reducer are the "worm" (or "worm screw") and "worm wheel". The "worm screw" is an "endless" screw held in place by a bearing at either end allowing the screw to turn freely. This "worm screw" mates with the "worm wheel" which is a type of "helical" gear. Typically the "worm screw" is manufactured from high carbon steel and the "worm wheel" is manufactured from "brass" or "bronze". The size (number of teeth) of the wheel is what sets the ratio. In a typical "single-start" worm system, with each full 360° turn of the worm gear, the mating worm wheel only moves 1 tooth. So if we equate that to a worm wheel with 15 teeth, the ratio of this combination will be 15:1. As you can imagine, with normal ratios used today, the right-angle worm reducer has a distinct size advantage over the helical gearbox when it comes to size... with the same final ratio.
But everything comes with it's problems, right? While all gearboxes require lubrication, it is extremely important in the right-angle worm gearbox. The "sliding" action of the worm screw along the teeth of the "worm wheel" causes more friction than the mating of the helical gear set. That's one of the reasons for the worm wheel being manufactured from a "softer" brass or bronze material. The softer metal acts sort of like a lubricating feature. But, bearing this all in mind, it's important to note that the mounting position of the right-angle worm gearbox is somewhat critical. Manufacturers have done a good job of engineering the lubrication system, but still, not all worm gear speed reducers like to be mounted in all positions. Mounting one on it's "end" can sometimes be a problem. So just make certain you check the manufacturer's installation instructions for any specific notes on detrimental mounting positions.
Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater. But higher ratio worm gear reducers become inefficient. For instance, a 5:1 ratio worm gear may have an efficiency rating of 95%. But when one needs a 75:1 speed reduction, and selects a right angle worm reducer, the efficiency rating may drop to 50% or less! And this causes significant power issues. You may have to increase the input horsepower to overcome the losses due to gearing inefficiencies. But looking at the same scenario with helical or helical-bevel gearing, one expects to lose 2-3% efficiency per gear set. So that means that if we can get the 75:1 speed reduction in a 3 stage helical gearbox, we may lose 10% or less efficiency. Much better than 50% loss.
Most worm gears, especially higher ratio combinations, have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place. This feature is useful for machines such as conveyor systems, in which the locking feature can act as a brake for the conveyor when the motor is not turning. For this same reason, depending on the ratios involved, some right-angle worm gearboxes cannot be run in the "reverse" direction. The physical mechanical properties of the worm screw simply won't allow it to happen. Be cautious if your application requires that the process be operated in BOTH directions. The gearbox may not allow it to happen.
Right Angle Gearboxes refer to such types as Worm, Bevel, Helical Bevel, Helical Worm, Planetary, Spiral-Bevel Gearboxes. The difference being how the gear sets are cut as well as combining different styles in different stages of a single gearbox. We'll discuss them in a little more detail in a sister topic.
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