Friday, 25 December 2015

WORM AND WORM GEAR

WORM AND WORM GEAR



Now we have understanding of Bevel gear and it’s classification that we were discussing in our previous post, where we have discussed various types of bevel gears used in industries and also the basics of bevel gears. Now we will concentrate about two important types of gears i.e. worm and worm gears used in industries for transmitting the power and motion at various speed with different ratios of speed.

Worm and worm gear will work in one unit to transfer the power and motion from one shaft to other shaft and these two shafts will be non-intersecting and non-parallel. A unit of worm and worm gear will have ability to provide the speed ratio up to 300:1 also and that is why we select worms and worm gears where we require high speed reduction. 

Usually worm will drive the worm gear and hence worm will be considered as driving member while worm gear will be considered as driven member. There is one interesting point to be noted here for worm and worm gear unit that is worm and worm gear will have teeth of similar hand. If we are procuring a set of worm and worm gear, supplier will provide us the worm and worm gear having right hand teeth until unless we are not specifying  that we need set of worm and worm gear with left hand teeth because right hand teeth is considered as standard for worm and worm gear set.

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Picture showing a set of worm and worm gear as mentioned here 

Many worms and worm gears will have one very interesting characteristics and that is worm will drive the worm gear easily but worm gear will not be able to drive the worm but we must have to mention here that this characteristics is subjected with shock and vibration.

In order to avoid the reverse motion, friction angle must be higher as compared to lead angle of worm gear. Usually lead angle of worm gear will be too small that friction between worm gear and worm will hold the worm if worm gear will try to rotate the worm. But if co-efficient of friction between worm gear and worm drops due to shock or vibration then in that case friction angle will also be dropped and if it is dropped below lead angle of worm gear then in that case reverse movement might be possible. 
If i am wrong, i will request to my readers to suggest and correct me by giving comment in comment box below. 

Let us see where we need to prefer the worm and worm gear

There are following points which could be secured easily by using worm and worm gear mechanism
  1. Restrictions with space i.e. we need higher speed reduction in compact space
  2. Larger speed reduction ratio
  3. Two shafts need to be connected to transfer the motion and power and these shafts are at right angle, non- intersecting and non-parallel.
  4. Output torque requirement is high
  5. Initial capital is low
We must note it that a worm and worm gear unit must have similar pressure angle, lead angle, tooth dimensions , teeth hand and pitch to work for  securing the above desired requirement.

Lead angle in worm

Lead angle is basically defined as the angle in between a line normal with the axis of worm and a tangent line with thread at pitch diameter. Lead angle is the most important term when we wish to determine the efficiency of worm and worm gear unit. Efficiency of worm and worm gear is directly proportional with lead angle i.e. efficiency of worm and worm gear unit will increase with increment in lead angle.
Image courtesy: Google

Lead angle of worm could be secured by using following formula. 
Tan ϒ = Z/q

Where, ϒ is lead angle
Z= Number of threads i.e. starts on worm
q= Diametral Quotient
Diametral Quotient, q= d/m
Where d is pitch circle diameter of worm gear and m is the module.

Application of Worm and worm gear

There are various applications of worm and worm gear as mentioned below
  1. Worm and worm gear units are used in long travel drive mechanism of barge unloading or ship unloading crane.
  2. Worm and worm gear units are also used in conveyor drive mechanism
  3. Worm and worm gear units are also used in automobile steering mechanism
  4. Worm and worm gear units are also used in hoisting equipments
  5. Worm and worm gear units are also used to control the gate mechanism in various industries especially in cargo unloading port sectors.
  6. Worm and worm gear units are also used in lifts and elevators
Do you have suggestions? Please write in comment box.

Also read

Pressure angle in gears
Gearbox inspection procedure
How to inspect a gear box
Gear noise and vibration 

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