Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why cant i get the same torque from a gear drive as apposed to a chain drive?

  1. Jan 4, 2010 #1
    I am trying to lift a load with a drill motor. I have a chain drive with a 3.7x1 ratio - 13 teeth on the drive gear with a 2.5" diameter and 48 teeth on the driven gear with a 7.75" diameter. It lifts the load easily.

    I also have a geared drive (no chain) with a 4.0x1 ratio - 16 teeth on the drive gear with a 1" diameter, and 64 teeth on the driven gear with a 4" diameter. The latter won't begin to lift the load. Whats wrong?
    Last edited: Jan 4, 2010
  2. jcsd
  3. Jan 4, 2010 #2
    Are you sure the gears aren't binding due to improper mesh? Otherwise other than friction and binding I don't see the problem.
  4. Jan 4, 2010 #3
    I can turn the geared drive freely with my fingers using my other hand to apply a load
    Last edited: Jan 4, 2010
  5. Jan 4, 2010 #4
    Do you think the gears increase the rotational inertia in the driveline? Or the stiction force when starting the load? These two factors impact the sufficiency of starting torque.
  6. Jan 4, 2010 #5
    I know this sounds painfully obvious, but we all forget/ignore the obvious sometimes...
    The drive gear on the geared system (assuming there is no intermediate gear) will have to turn in the opposite direction of the chain-drive system to move the load in the same direction. You will need to reverse the drive gear rotation when switching systems.
  7. Jan 4, 2010 #6
    I did reverse the direction of the motor for the geared unit, if I get it to work I will install an intremediate gear. System Theory you are above my head on that one I do know both units are starting from a standing state.
    Last edited: Jan 4, 2010
  8. Jan 4, 2010 #7
    Do you know the mass of the load? Are you lifting it with a cable on a drum? If so what is the drum radius? How thick are the gears you're using and are they heavy compared to the chain and sprockets? (Are the sprockets relatively heavy or light?)
  9. Jan 4, 2010 #8


    User Avatar
    Homework Helper

    There's not enough information. You've given us enough information to calculate relative torques, but not linear forces. How is the torque at the driven gear being converted into a linear force to lift the load?

    If both the 7.75" sprocket and the 4" gear are turing the same diameter drum, then there's a second torque multiplier involved, and this torque multiplier would be larger for the 7.75" sprocket.

    Lastly, chain drive losses are less than gears. Effenciency might be about 92% with chain, and about 87% with gears.
  10. Jan 5, 2010 #9
    Based on what you all are saying, could my problem be the difference in the sprocket diameters and the total number and size of the gear teeth compared to the chain sprocket? If that is probably the cause I will buy a new pair of gears closer related to the chain drive. The load is lifted with a cable drum I will find the diameter if it is now needed. The approximate weight of the load is 200-300 lbs.
  11. Jan 5, 2010 #10
    As Jeff said the radius of the drum on the take-up cable is relevant to the torque leverage. It should be possible to caculate the minimum starting torque required if the drum radius is known.
  12. Jan 14, 2010 #11
    I used a torque wrench to find the starting torque. It took 250" lbs to initiate any movement of the load. I measured the max torque on my drill motor and it was approx. 150" lbs with a fully charged battery. It seems that a four to one ratio should be sufficient but you may know additional forces that apply. I guess I will just go to about a six to one ratio, do you think that would work? Thanks again for all your help
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook