Tranny Ratios - What am I missing

[Neil Kozlowski]

According to the ZF manual, a ZF-10 with a ratio of 2.045 to 1 can handle 38 hp at 3000 rpm. Additionally, a ZF-10 with a ratio of 2.722 to 1 can only handle 27 hp at 3000 rpm and if the ZF-10 has a ration of 1.482 to 1 it can handle 44 hp at 3000 rpm.

I would have thought that the greater the reduction the MORE power it could handle. I obviously am missing a key point.
Can anybody help in explaining where I have gone astray?

Thank you!

[Parfait's Provider]

It is late, but if my mind is at all clear the ratings are probably based on the internals of the tranmission and transmitting more power at a faster rate is definitely going to take a beefier structure than low power at a low rate. So for the same internal components, if you raise the rate, you must lower the power and vice-versa.

[Neil Kozlowski]

I'm not sure I follow you. The higher the reduction ratio I would think would make it easier ( a bigger mechanical advantage), therefore. more power can be transmitted. Kind of like a two speed winch! Obviously I'm incorrect, but I still don't see it.

[Tod Mills]

Hi, Neil

One thing to bear in mind is that power is a function of torque and speed (rpm). For a given power transmitted, as the speed goes down, the torque goes up. As the torque goes up, the stress on the teeth of the gears in the transmission at the output end goes up, so to maintain an acceptable tooth stress (and gear life), the power has to be reduced.

There are probably more factors than this that come into play, but I expect that is the main one.

Less than two weeks to Buzzard's Bay! woo hoo!

[Oswego John]

According to the ZF manual, a ZF-10 with a ratio of 2.045 to 1 can handle 38 hp at 3000 rpm. Additionally, a ZF-10 with a ratio of 2.722 to 1 can only handle 27 hp at 3000 rpm and if the ZF-10 has a ration of 1.482 to 1 it can handle 44 hp at 3000 rpm.

I would have thought that the greater the reduction the MORE power it could handle. I obviously am missing a key point.
Can anybody help in explaining where I have gone astray?

Thank you!

Hi Neil,

As usual, I could be wrong but here's my slant on the subject.

I don't think that you have gone astray. I do think that we have to shuffle things around a bit before we tackle the problem. Then, by agreeing on the wording, we can all be on the same page. We have to agree on certain terminology in order to proceed. (Sheesh, what a fuddy duddy)

First off, I would suggest that the reduction gear ratios be listed in order along with their suggested horse power. The lowest ratio first, then the mid range and lastly, the greatest reduction ratio.

1.482 : 1 @ 44 HP @ 3,ooo RPM

2.045 : 1 @ 38 HP @ 3,000 RPM

2.722 : 1 @ 27 HP @ 3,000 RPM

I'd like to be sure of the terminology being used. Does the manual actually say "a ZF-10 with a ratio of 2.045 to 1 can handle 38 hp at 3,000 rpm"

Since we are talking about a reduction gear, I assume that the input rpm going into the tranny will be higher than the output prop shaft rpm.

I think that it's safe to state, internal, inherent losses not withstanding, that work in is equal to work out. Effort/rpm in is equal to effort/rpm out. There is no free lunch when considering mechanical advantage.

Suppose that your niece is sailing your daysailer on a breezy day. The pressure of the sail on the single sheet is 100 lbs. It's too much for her to control it. You now install a four part set of sheet blocks. To handle the 100 lbs sail pressure, she only needs to exert 25 lbs on the blocked sheet. But wait, there's no free lunch. For her to haul the boom in five feet, she has to haul in five feet times four (block ratio) or 20 feet of sheet line.

But what about the size of the motor compared to the mechanical advantage (reduction gear)? Your niece wishes to raise a box that weighs 100 lbs for a distance of one foot. She plans to use a lever with a mechanical advantage (MA) of 1:1. The lever is one foot on either side of the fulcrum. She can't budge it.. You hand her a lever with a 5:1 MA Five feet on her side of the fulcrum and one foot on the box side. By exerting only 20 lbs of pressure, she can raise the 100 lb box. But wait, no free lunch. Foot/lbs input equal foot/lbs output. She has to push down on the lever with 20 lbs effort for five feet to raise the 100 lb box one foot.

With a low reduction ratio of 1:1, an athlete or strong man (larger 44 HP engine can raise the box. With a higher degree of reduction, 2.722:1, a smaller person (27 HP engine) can raise the same box.

The next bit of anology can lose some people, but try to stay with it.

Gear ratios can change. (Consider a standard shift on a vehicle.) The ratios can increase or decrease. On a standard bicycle, there is a large chain sprocket in front with pedals attached. On the rear wheel is a smaller sprocket. This setup is the opposite of a reduction gear ratio. It increases the ratio past negative zero and into the positive range. For the sake of argument, let's say that the increased ratio is four to one. For each revolution of the large front gear, the rear gear will revolve four times. This bicycle happens to have an interchangable derailleur gear set up.

In your mind, compare this bike to your boat. The front sprocket will be your engine, the rear sprocket will be the prop shaft. Through gearing, they both rotate at different speeds from their input rpms.

While pedaling across the country, going through the flatlands of Kansas, your niece has no problem using the 4:1 gear ratio. Later on, she hits the foothills of the Rockies. She has to down shift, reduce the gear ratio. Finally she is climbing the Rocky Mountains themselves. She is now pedalling in low,low.

But wait a second Mr Postman. Wasn't one of the stipulations of the original theorem that 3,000 rpms was to be maintained for all situations? Let's transpose the 3,000 rpms of the boat engine to 30 rpms for the front sprocket. This means that no matter what the gear ratio is for the rear sprocket, the pedaler, your 100 lb niece must maintain 30 rpms on the front sprocket even while going up the side of a mountain.

This pace might be too difficult for her. With that in mind, maybe while in this lower gear ratio, a stronger cyclist (larger engine) might be able to maintain the constant pace that is called for in the equation.

That's enough for tonight. I'm getting a headache. (Two Tylenol)
Later,
O J

[Didereaux]

(deleted)
That;s the way I am reading anyways. I obviously didn't read nearly enough, no props involved at all....sorry! ;(

g'Luk

[Parfait's Provider]

Is your two-speed winch easier to crank with a higher ratio or a lower ratio? Lower. When you crank with it in high, where does the winch transmit the force that isn't converted to drum rotation, like when you can't crank anymore? To its bearings, the teeth of the gears, etc. Very low RPM, but high destructive forces. In other words, it is possible to overpower the winch and tear it apart without doing a lick of useful work. Same thing applies to your transmission. It has been designed to an HP and RPM rating; if one goes up, the other has to go down or the life of the transmission will be adversely affected.

You might also want to keep in mind that engine manufacturers sometimes skimp on the transmission in order to keep the package price down. Don't abuse it or you will be having very expensive problems.

Now I am going to read OJ's comments.

[Parfait's Provider]

that if your niece's bicycle is built for her to use efficiently, my 230#s will destroy it in short order unless I limit the force on the pedals and don't stand on them even when coasting down a hill.

It is a bit counterintuitive, but be assured that if the manufacturer could claim more for his transmission he would.

[bill2]

OJ

Just returned from a little boating sojourn and couldn't pass up the opportunity to comment heh heh . . .

Cool - maybe you're gonna invest in a little bike shop in 10 - 20 years . . .

Good explanation






.

[Oswego John]

bill2

Hey Bill, if you could guarantee that I'd be around for 10-20 years we could both open a bike shop together in a heartbeat. You can be pres. and vice pres. I'll be the treasurer.

Just one thing, though. We'll make a fortune making bikes for boys and girls but will lose our shirt making bikes strong enough for 230 pound southern sea captains. helloooo? Anyone home?

O J

[Parfait's Provider]

OJ,

I guess we just had to hijack this thread for something more entertaining?

Mountain bikes are pretty rugged and we old heavy southern sea captains probably can't make them do all the tricks young whippersnappers try. They might even survive and you and Bill would thrive.

Has Winter yet set into that northern Paradise?

[bill2]

Well since we're "pirating" this thread . . .

So around 45 years ago - dad - who was an aspiring engineer and I still young enough to be sticking my nose into everything - wanted to chuck it all and open a rental/repair bike shop in Montauk with my three uncles. Iffen it woulda happened I 'd probably be able to afford ( Montauk changing like it did ) both a bike and a cape dory - well the bike Ok but the cape dory is ( sweat ) equity - which of course is tonic for the soul.

Now OJ if we could use the bikes ( maybe mtn bikes to hold up to the tourists ) to pay for the cd's - now we'd have something - and sure you can hold the money - ( I once rode my bike to Montauk from my aunts place and the money doesn't last long in Montauk . . . )