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Some pulley math

N

notjustair

Well-known Member
I bought a ZTR that a guy was going to junk because he couldn't get it to work. New carb kit and it's running fine. I noticed that the deck seemed to to put an unusually big load on the engine. The motor is relatively new and the deck spindles were free.

There were new pulleys everywhere and I noticed that the engine pulley was 4" and the deck pulley was 3 1/4". I called the company and they are supposed to be 4", thus the overload.

So, can any if you smarties tell me how much faster the deck was running? If I put a 3 1/4" pulley on the engine would it have the same result? If the pulleys are the same size does it matter how big they are?
 
4" drive pulley/3.25" driven pulley = 123% speed increase

Any equal sized pulleys would be 1:1, or equal speed
 
I think that bigger pulleys are easier on the belts and also are less prone to slip, but that is just based on my own experience and I have no data to back it up. I don't suppose the extra 3/4" would make any appreciable difference anyway in terms of the belt being happy, so I would think either size could be fine. The circumference of the 3.25" pulley is 10.2 and the 4" is 12.56 according to my calculation.
Zach
 
The broader the curve that the belt has to go around, the less bending/flexing it has to do.

Also, the larger pulley gives the belt more contact area, resulting in less slippage and better transfer of power. The belt will definitely last longer.

It's a matter of replacing one pulley or three, right? How much adjustment do you have on the belt? Will you need to buy a new belt as well?
 
output rpm = input rpm x drive over driven. so if your engine rpm is 3500 x(4/3.25) = 4308 rpm
v/s 3500 rpm if pulleys where same size so 808 rpm faster or 23%. Idealy the larger dia pulleys would be better from a belt wrap/slippage perspective, but that is not to say 3.25 wont work? Also od of pulleys are not what you use to calculate exact rpm, it is the "pitch dia" but for all intensive purposes in your case OD is close enuff
 
pulleys the same size equals same rpm---but the larger the pulleys the more torque/horsepower you loose.
 
Actually the horsepower stays about the same. The torque drops as the rpm increases, just like shifting gears.
 
I think dpendzic is talking power loss due to friction of the belt on the pulleys. However, bending the belt more on a smaller pully (tighter radius) also costs power, so it might be a wash (depending on the belt, the position of the moon, rain totals, etc...).
 
Before you start changing out pulleys, have you tried sharpening your blades? Ultimately, it DOESN'T MATTER how fast your blades turn because the amount of work the engine has to do is determined by GROUND SPEED, not the speed of the deck spindles. If you are going five MPH, you are cutting the same amount of grass whether your blades are turning at 3600 rpm or 4400 rpm. The faster blades will fan more air, but I wouldn't expect that to be noticeable.
 
You probably should calculate the blade tip speed. I poked around to see what various ZTR manufacturers publish for spindle speeds and found nothing. But Exmark publishes blade tip speed for all their mowers, and it looks like they shoot for 18,500 feet per minute across the board. A couple of examples:

16.25" diameter blade at 18,500 fpm: (18500 x 12)/(16.25 x 3.1415) = 4350 rpm

21.6" diameter blade at 18,500 fpm: (18500 x 12)/(21.5 x 3.1415) = 3270 rpm
 
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