Belt and Pulley Gear Reduction

Discussion in 'Frame Mounted Engines' started by skyl4rk, Jan 2, 2009.

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  1. skyl4rk

    skyl4rk Guest

    What is the smallest pulley that you can use on the motor shaft?

    If you were to use a 1/2" wide v-belt (standard A size), can you get by with a 2" pulley?

    This would be for a system with a slip belt / idler pulley clutch.

    The 2" pulley would match up with a 10" pulley for 5:1 reduction.

    Huasheng 50cc four stroke, 26" wheel, overall reduction 18:1.

    Anybody smell belt smoke?

  2. skyl4rk

    skyl4rk Guest

    Here is a parts list for a Huasheng 50cc reduction gear:
    (Serial numbers are

    If can make a centrifugal clutch to fit the Huasheng 50 and this pulley:

    Engine Pulley: Variable pitch v-belt pulley 1.9" - 2.9" OD, 5/8" bore, 3/16" keyway, #6205K112 $25.48

    Then use the clutch, otherwise, use an idler pulley and slip belt "clutch".

    Jackshaft: 5/8" shaft, 9" long, 3/16" keyway, #1497K108, $18

    Mounted Bearings (2): 5/8" shaft, #5913K42, $22 ($11 each)

    Jackshaft input pulley: 10" OD v-belt pulley, 5/8" bore, 3/16 keyway, 1/2" belt width (A), #6245K952, $23.21

    Jackshaft output sprocket: 11 tooth, #40 chain, 5/8" bore, 3/16" keyway, #6280K652, $9.70

    Match with a 44T or larger sprocket on wheel.

    If you can't get a clutch to fit, use an idler pulley:

    Idler pulley (if needed): Flat belt pulley 9/16" width, 1-3/4" OD, 1/2" bore, cast iron, #1355K2, 28.65

    The variable pitch engine pulley should give you a range from 26mph to 39mph at 7000 rpm with a 44T wheel sprocket.

    It may be desirable to go to a larger sprocket, 48T or 52T to allow the engine pulley to be larger.

    I would make the engine mount out of angle steel and start out by mounting the motor to it, then add some plywood to keep the two angle irons parallel, then mount the jackshaft. Use plywood, solid wood and u-bolts to attach to frame.
  3. ocscully

    ocscully Member

    Could you possibly provide an explanation how the variable pitch pully works? I'm not understanding just exactly how it functions to provide the range of diameters?

  4. skyl4rk

    skyl4rk Guest


    Variable Pitch: Bored to fit directly onto a shaft through a standard keyway and a set screw. All have standard keyways and set screw, except 1/2" bore size, which has a standard set screw. Speed can be varied by up to 25% by moving the sides of the pulley together or apart, thereby adjusting the pitch.

    On page 1026:

    Adjust the pitch of these pulleys by moving the sides together and apart on the threaded hub. This adjustability provides maximum surface contact with the belt for more efficient driving and longer life. Use pulleys only with V-belts listed below.

    I have not seen one. I assume that you can adjust it with the belt off. Possibly the pulley must be removed to adjust the pitch.
  5. jimraysr

    jimraysr Member

    Adjustiable Pitch Pulley

    The movable part of the pulley has a set screw and the threaded part of the pulley has a flat. Remove the belt, loosen the set screw, turn the adjustment to the new setting and tighten the set screw to lock it on the flat.

    The adjustable pulleys are very common in this part of the country for evaporative (swamp box) coolers.

    I have an antique trencher that uses an idler pulley for a slip clutch. I would guess the belt is around 20 years old? The engine pulley is 3" and the driven is 10". Then it has a 3" sprocket wt. the 10" pulley and a 10" sprocket on the digger shaft. Quite a drive train. ) That is telling me that the belt can handle the torque for the primary reduction and clutch duty, after a 3.33 increase in torque, they needed a chain to handle the torque of an other 3.33 increase?

    I don't remember the nominal horse power of the 40 year old Briggs, but guess around 5 HP at 3600 RPM.

  6. skyl4rk

    skyl4rk Guest

    Thanks, I am thinking that 3" is a better choice for a engine pulley diameter, maybe moving up to a larger size variable pitch pulley, something like 2.5" to 3.5" diameter.

    Idler pulley - belt slip clutch is a pretty ancient technology. I would probably try to get a centrifugal clutch with a variable pitch pulley on it. It should be available but apparently the Huasheng has an odd shaft and it is not easy to get one.

    I am tired of waiting for a decent frame mount gearbox. The sad thing is buying the parts for a belt gear reduction system is not much less than a kit with a gearbox, you are not really saving any money by making it yourself. At least you have the satisfaction of a homemade system and you know who to blame when things fly off.

    I'm going to make a parts list and then try to start looking around to scrounge stuff.

    So here is the gear reduction, starting with a Huasheng 50 with the shaft on the riders left side of the bike, and ending up on a sprocket bolted to a 26" wheel on the riders left side of the bike.

    3" pulley to 10" pulley: 3.33
    11T sprocket to 44T sprocket: 4
    Overall reduction: 13.32

    This needs more reduction, so here it is with a larger 60T wheel sprocket:

    3" pulley to 10" pulley: 3.33
    11T sprocket to 60T sprocket: 5.45
    Overall reduction: 18.2

    6000 rpm is in the middle of the Huasheng 50 power band and would be a good cruising rpm.
    6000 rpm / 18.2 = 330 wheel rpm at cruising speed
    Wheel circumference: 26" * 3.14 (pi) = 82"
    82" x 330 rpm = 26400 inches per minute
    26400 inches per minute / 12 inches per foot = 2200 feet per minute
    2200 feet per minute / 5280 feet per mile = 0.42 mile per minute
    0.42 miles per minute x 60 minutes = 25 mph
    25 mph is a great cruising speed, maybe even a little high for my old cruiser bike.

    With a variable pitch engine pulley I could reduce the pulley diameter a bit to slow down the top end speed and improve low speed acceleration.

    With a 2.75" engine pulley, cruising speed at 6000 rpm would be 23.5 mph.
  7. jimraysr

    jimraysr Member

    Home Built Drive System

    I love building drive systems. First was a horizontal aluminum 2 stroke engine mounted on aluminum honeycomb panel from a B-47 driving a 12 volt generator to charge my battery when out on Search and Rescue missions in early 60's. It really was a generator as alternators weren't common then. Had to mount the generator facing the engine as it needed to turn CW and the engine turned CCW so a parallel drive wouldn't work.

    I would look to equipment rental places for possible sources of drive materials. Haven't been in a lawn mower shop in a long time, but don't think there are many residential mowers wt. belt drives anymore. I gave a Tiff mower away last year and it had a slip belt drive for the reel and the drive roller was a chain which always turned, but you let it down when you wanted forward drive. Those kind of mowers would be a source of parts.