Pulley math

[AldeanFan]

Need some help with pulley math, this is something I know nothing about so please explain it like I am in Kindergarten.

I’m doing a pulley swap on a lawn tractor to give it a higher top speed.
Stock engine pulley is 3-1/2” and driven pulley is 7”

if I go to two same size pulleys how much faster will my top speed be?

 

[Steve W.]

Theoretically, twice.

However, it also depends on whether the engine has the power to do it.

For example: some cars (or motorcycles) with lower power output and have five- or six-speed transmissions will often go faster in the next-to-the-last gear than they will in top gear. The top gear is more for cruising than it is for top speed.

Sorry, forgot the explaination. Given your sizes, the drive pully has to rotate twice for every rotation of the driven pulley. With two pulleys of the same size, they will rotate at the same speed.

How much faster do you really need to go? Twice as fast might be too much.

.

 

[AldeanFan]

Thanks
I may have to experiment with different pulleys to get exactly what I want, right now top gear wide open is barely walking speed.

 

[jack stand]

I've seen online calculators for pulleys and chain sprockets.... somewhere. I'd bet a quick search would get you several choices.

 

[Jeff Ivers]

Multiply the pulley diameter in inches by 3.14 to got the distance the belt/chain travels in one revolution around the pulley. Do this for each pulley and then divide the distances to get the ratio.

 

[The Cobbler]

good calculator here https://www.blocklayer.com/pulley-belteng.aspx

upsizing to 7" from 3-1/2 will theoretically double your speed . but it may labour the motor so it's rpms could be less and affect top speed

 

[laser3kw]

Thanks
I may have to experiment with different pulleys to get exactly what I want, right now top gear wide open is barely walking speed.

Is this a gearbox type or a hydrostatic drive? Hydrostatic drives don't like to be over sped much.
your bigger problem will be getting the correct belt length. You may have to buy a couple of different lengths to find the "goldilox" belt.

 

[BMW Rider]

Multiply the pulley diameter in inches by 3.14 to got the distance the belt/chain travels in one revolution around the pulley. Do this for each pulley and then divide the distances to get the ratio.

While this method will determine the ratio correctly it is unnecessarily complicated. The same result will come from simply dividing the pulley diameters. In your case: 7 divided by 3.5 gives you a 2:1 ratio which translates to mean for every 2 rotations of the drive pulley the driven pulley turns once. For equal sized pulleys the ratio would be 1:1 which as others noted is double the speed when not accounting for other factors.

Once you figure out what pulley sizes you want to use, an easy way to find the belt length is to simply use a string or tape measure wrapped around the pulleys. Belts are measure by their total length if you were to cut it and lay it out straight and the length is usually indicated in the part number.

 

[Monza Harry]

AF, what kind of tractor and transmission/transaxle? Some older tractors with a Peerless trans, had a low range, about 2:1 reduction IIRC these tractors would pull way outside there weight class so to speak but in low range were pretty slow. The range shifter can be in a nonintuitive position so search arounda little. Just a thought if this is a non molested original. Also some older units have had a non traditional shift pattern, so check that you are indeed in high, not a dig in any way, there are some strange things I have seen and I haven't been very deep into the oddities, but a little. Harry

 

[txvwnut]

Do you have two pulleys on the crank, one for chassis and one for pto or one pulley. If it’s only one pulley then you may run into an issue with blade tip speed on the cutting deck.

 

[nadogail]

The Pitch Diameter of a pulley is something less than the Outside Diameter. But the concept of determining the ratio between the pulleys is correct.

 

[AldeanFan]

Thanks everyone for the advise.
The 1/2 size pulley makes it go very fast but it is almost impossible to get moving in 2nd or 3rd without stalling, so I might try a slightly larger pulley.

this is on a 1969 John Deere lawn mower/tractor that I turned into a mini Jeep for driving around the pits at the racetrack.