Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Quote from: Jim on April 12, 2021, 08:11:18 pm

Quote from: St Paul Steam on April 12, 2021, 08:05:51 pm

Why not something like the D32 has ?

I've been studying that mechanism Bruce.

I always wanted to emulate the clutch system on an old wheel horse lawn mower with a horizontal shaft engine. I tried to find a video on YouTube but was unsuccessful.

Can't picture what that looks like Bruce.

Sorta' follow that....but way out of my league I reckon Bruce.

At first I thought of turning a cone and an inverted cone like below, but anything like that is going to be have to be turned by the engine as well and will eat up engine energy (and there's little already with most of our engines).

So far I'm thinking the idler wheel as suggested may be the way to go, but not sure how to do that either.

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Good Morning Jim

Here's my attempt at depicting a typical Idler Pulley tensioning system, that can act as a clutch within a common belt and pulley drive system.

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So let me describe what I'm trying to represent here.

On the left is the engine's crankshaft pulley, or what might simply be called the power pulley. On the right is the pulley that is to be driven, such as on the shaft of your Baker fan. Clearly in this case we are looking at a reduction gear drive train, in that the power pulley is much smaller than the driven pulley.

The pulley shown hanging low, deflecting the lower leg of the belt, is the idler pulley, and it is there simply to draw the drive belt tight enough to create the needed friction to engage both the driving (power) and driven pulleys.

For simplicity, I show the arm holding the idler pulley as pivoting on the driven pulley's shaft, or in your specific case, the Baker fan's shaft. But the idler pulley arm could be mounted on the Baker fan's frame, base, or on the engine frame or base, or even as a stand alone unit in between .... but that adds complexity.

Now for some important caveats that must be understood for any of this to work properly.

As depicted in my pathetic sketch, the system is set up for counter-clockwise rotation. If your engine needs to run in a clockwise direction, then the idler pulley would best be deployed on the upper leg of the belt.

Let me explain why that is, and give you the general concept that is behind it.

The general theory of critical rope function (chain function too BTW) is that a rope only transmits energy while under tension .... which is to say that you can only PULL things with a rope, but you really cannot PUSH anything effectively.

Your belt being used to transfer power from the engine pulley to your Baker fan pulley is just like a rope, such that when your engine is rotating counter-clockwise, only the top leg of the belt is under tension from the power load, and is thus the portion of the belt that is transferring power to the fan. The lower leg of the belt is really just a return loop and only needs to be tight enough to provide grip on the pulleys. Thus, by placing your engine and fan a little too close together, so there is too much slack in the belt, the pulleys will not get enough grip on the belt to drive a load.

Here then, enter the idler pulley, which can provided the needed tension in the belting for it to start gripping the pulleys and driving the load.

Now, should your engine rotate clockwise, all of the above is reversed, so the upper leg of the belt becomes the slack return leg, and the lower leg is the tensioned drive leg.

The reason you generally want to have the idler pulley ride the slack leg of the belting, is that on the power leg, the first effort of applied energy will be to try to straighten the belt between power and drag, thus lifting your idler pulley out of the way defeating its purpose, unless it is mechanically fixed in place. But generally idler arm pulleys are only held in place by gravity or spring tension, which wouldn't work well against much power input.

Hope any of that helps, but I suspect I've likely further muddied the waters, so feel free to ask questions of any part I didn't describe well enough.

You are correct as far as it goes Wayne. But in this instance there is no significant power being transferred and the outward deflection will work fine so long as the angle to either of the pulleys doesn't get too shallow. Granted squeezing the belt toward the center does provide for more degrees of contact around the pulleys, but perhaps not too many more depending on amount of deflection in the system. At the same time, the set-up I have sketched is not to be taken as truly proportional, but rather just representational. Less deflection of the belts than my sketch shows would be desirable. Also note that should the set-up I've depicted be used for a clockwise running system, the idler pulley would be outside the belt loop pushing down (inward) on the upper belt, in a configuration similar to what you show, only with the opposite direction of motion.

The fan belt on my old Toyota pickup is set up much like my sketch shows, with the generator acting as the adjustable belt tensioner (equivalent to an idler pulley), in approximately an equilateral triangle configuration, which is actually much more open than what is shown in my sketch.

Meanwhile, you are showing your idler on the tension side of the belt, where it will try to kick out harder, rather than on the slack side of the belt where it will be mechanically easier to add the needed tension, whichever way the belt is deflected.

G'day all and a massive thank you for all the thoughts and ideas.

Just got to keep reading through and digesting them all. Appreciate the solutions & suggestions proffered up.

Looks like fun in the sun in bygone days from here!

I would note that it appears your idler pulley is on the slack side of the belt in the photo, which is opposite to how you sketched it yesterday ..... a much better arrangement to my way of thinking.