Donkey Engine

[Prospect]

Hi, I built a Donkey Engine based on the William Harris book, only increasing the size by twofold. I.ve got tired of small fidelly parts. Truth be told that for health reasons I can't feel small parts any more. That being said. The engine has been completed for over a year and I've been contemplating the boiler. I,ve got some conflicting info from our local boiler authority and local museums and commercial builders, so I,ve waited and contemplated. I do have a copy of the Australian Miniature Boiler Code which I think is very well done. I,ve draw in 3d Cad a boiler based on 2x the boiler dimensions in the book adjusting for the size of readily available boiler grade materials. I would appreciate your opinions and expertise on the boiler. If OK I'll post my questions in the boiler section. I could post the 2d pdf file of the boiler but I don't know if this is a good thing or not. Liability? Attached is one picture. I would or will post more for clarity when I figure out how to reduce the size of the rest to exceptable sizes. so here goes. Thanks, John Sorry just thinking. Just some info on myself. I got my first metal lathe when I was 10. This was courtesy of a well intensioned Public Trustee and my parents acceptance. Little did the Public Trustee know that She steered me to the next 47 years of joy of model engineering. I farm in the real world. I have built a 1x 1/2" Pacific Loco. and numerous small engines. Thanks again, John

I should have mentioned that the boiler in the picture is a dummy. It's just sheet metal wrapped around wood discs the right diameter so I could make some of the fittings such as the firedoor. John

 

[dalem9]

Welcome John glad your here. Dale

 

[JorgensenSteam]

Wow, very nice donkey engine.

Welcome.

Pat J

 

[Maryak]

John,

Welcome to our forum. wEc1

Best Regards
Bob

 

[ShedBoy]

Hello, Nice Donkey. Would love to see some of your other engines

Brock

 

[ref1ection]

I have to agree, would love to see all of your other engines. The donkey looks fabulous.

Ray

 

[Prospect]

Thanks guys. When I figure out what I'm doing wrong with my camera I'll post pictures of the two small engines in my office. My Locomotive is packed away as being on a grain farm mice were packing every open space with stuff. You almost have to admire the little so and so's for their tenacity. John

 

[Prospect]

Just another couple of pictures with the dummy boiler. John

 

[b.lindsey]

Hi and welcome John,
Nothing small about that jewel at all..thanks for sharing the pictures of it.

Bill

 

[Prospect]

just a picture of two engines in my office. both were built from published designs. They need cleaning ( on a shelf in the I don't dust zone ). John

 

[Steve J]

John
my name is steve and am building the donkey engine. I just finished the guide plate (f9) and am confused about where it goes. On page 10 it shows it under the base plate and channel. Same on page 3 but how is it held in place? I have not drilled the channel yet but have drilled the base plate and guide plate.
thanks , steve

 

[Prospect]

Steve, On my engine the F9 guide plate bolts to the top of the channel. You can see that in the first picture I posted. When I scaled things up there was a clearance issue and the angled shape of the channel I used would have required either an angled shim or a flat machined on the underside. Before you drill anything though, check the cylinder centerline height compared to the guide assembly measurements to see if you have correct clearance if you bolt the guide plate to the underside of the channel. It sticks in my mind that there was an issue there, but it might just of been my misinterpreting the origional drawings. John

 

[Steve J]

John
so is the channel threaded or is it drilled through and fastened using a bolt and nut?
steve

 

[Prospect]

Steve, Mine is threaded into the channel. You could use a bolt and nut but then you would need an angled washer to level the surface depending how flat your channel is.

 

[Steve J]

John
Ithink I get it now. Sometimes you just have to skip a step and when you get further along it becomes clear. I wish you were closer. Iam a welder by trade (38yrs) and could weld up a boiler with little trouble. Did one for a friend building a loco a few years ago and we hydro tested it to 400 lbs with no problems. Thanks for the help.
steve

 

[Prospect]

Hi, I thought it would be interesting to have an idea of the pulling power of the winch on this toy. I think the formula would be engine displacement x pressure x engine speed x winch ratio divided by 1/2 the winch drum diameter (first wrap). Effectively (12.4c.i. x 100p.s.i.)*120r.p.m.)*4to1 ratio)/1/2 of 4in. drum diameter) which gives a pull of about 2600 pounds. Does that sound right? John

 

[Jasonb]

I would not have thought that engine speed came into the equasion.

J

 

[Prospect]

Jason, That's what I'm wondering about. I'm not sure if the equation is correct at all. Using speed, I get a pull of 2467 pounds. Without adding speed, I only get a pull of 20 pounds. Running on 100 pounds of air it will break a 1/4" nylon rope quite easily. I hope one of the math experts can help. John

 

[Dan Rowe]

John,
I think that it is torque that you need to calculate. A Prony brake is a simple device for checking the torque of an engine.

The max power of a steam engine is when the crank is at 900. The force on the crank arm is the cylinder pressure acting on the piston diameter, so the bore and stroke is needed but it is force on a lever not engine displacement that is in the equation.

Dan

 

[steamer]

I would approach it from the power perspective also.

T = 2 x PI x N x T/33000

N = rpm
T= foot pounds

If your brake is mounted to the drum, you can measure the torque directly and get the rope tension

Figuiring in the power will tell you have fast you can pull the load

What isn't figuired into your equation is the frictional loss through the bearings and gears...though small, they are significant.

The power dissappated will be proportional to speed so I suspect the gears early in the train to dissapate a good deal of power.
Slow speed gears , well made are pretty efficient, but at 2000 pounds or so of force....the frictional load on the gears in the final drive and the drum bearings has got to be large.

Putting the brake on the drum will tell you the real story.


Dave