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Gears Help

  1. Oct 5, 2007 #1
    I need to know which gear has a better power transmission efficiency/

    1. Worm gear or the Spur gear and why

    as well theres a design for a device that must take power from a high speed but low torque source and deliver it to a low speed, high torque shaft. the speed reduction cant be done wtih just a single gear. it has to be a combined system. which gear should be on what. should the worm set be on the low torque , high speed end of the reduction system or the high torque low speed why?

    i think that it should be on the high torque because the worm gear itself can transfer high torque and have less stress on the gears while the spur gear set would cause lots of friction between the teeth.

    thanks in advance for your help
  2. jcsd
  3. Oct 5, 2007 #2


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    Welcome to PF, Wah.
    There really isn't much efficiency difference between different types of gears, although there can be between different styles within the same type.
    Since a worm gear is irreversible, you have no choice as to how it is installed. The worm has to be on the high-speed end, and the roller or pinion that it drives is on the high-torque end. The mechanical advantage is far higher than it would be for a spur gear set-up of comparable size.
    You should look into epicyclic gears, as well, one example of which is the automatic transmission in a car. They're capable of ratios in excess of 1,000:1.
  4. Oct 6, 2007 #3
    so theres no power efficiency difference for the torque power between the spur gear and worm gear?
  5. Oct 6, 2007 #4


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    A worm gear has a lower efficiency than a spur gear. However, there is a mountain more of work to specifying and designing a gear than just the type of gear one uses. The majority of what you are looking for will depend greatly on what the actual tooth design will be.
    Last edited: Oct 6, 2007
  6. Oct 9, 2007 #5
    does a spur gears which can be mounted on parallel shafts,could be mounted on two crossed shafts which dont intersect?
    Does the worm,worm wheel (as a single stage)would give you the required speed?if yes,so why not?if spur gear it would be more noisy,but lower cost and higher efficiency and longer life,but needs more lubrication.
    but just my ques is( How can u install spur gear,instead of worm,worm wheel gear?would u change the shaft positions?)
    why wouldnt u choose helical gears?for the higher horsepower applications?
  7. Oct 12, 2007 #6
    does a spur gears which can be mounted on parallel shafts,could be mounted on two crossed shafts which don't intersect?
    NO, helical or hyploid gears are used in such situation.

    worm system gives u a lot more reduction than spur system, so actual problem is not noise or lubrication, its the reduction needed.

    obviously worm wheel cant be replaced with a spur just as such. design modifications are needed for such situation. spur is used with parallel shafts whereas worm is used in skew shaft positions
  8. Oct 12, 2007 #7
    ok,thank you ank_gl,im agree with u that the main difference which u can look at when ur required to choose a suitable gear box is the reduction ratio.
    just i had misunderstanding of wah28 problem,thought he wanted a single stage gear box,and he wanted to know which type of gears would be the best to achieve his high torque and low speed required,so-i wondered how a worm&worm wheel could be mounted in the same shats position of spur gears,but when i read again,i found its a combined system of gear box which had both types of gears.
    thanks for letting me undertand-:).
  9. Oct 12, 2007 #8
    For what it is worth, with spur gears the teeth roll over one another; with worm and worm wheels the surfaces slide against one another. This should give some idea of the friction losses involved. That is the way I see it.
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