scotch yoke?

[Anatol]

Hi all
what are your opinions on the (efficiency, reliability) of the scotch yoke? It seems a clever way to avoid making cross slides. What are the pros and cons? Should it be made of cast iron for wearing? I guess the rod could have a sealed bearing on the end, running in the yoke?

 

[GailInNM]

I like Scotch Yoke drives. They can be built very short giving a lower center of gravity which can be helpful for marine applications. If you make the valve gear drive a Scotch yoke also it gives you a true sine wave motion which makes timing very easy. Main down side is they don't look as elegant as conventional crosshead designs. No appreciable difference in efficiency.

I built my first one some 35 years ago and later sold it as a kit for about 15 years. Sold the kit business. but the kit is still in production. It was 1/2 bore X 5/8 inch stroke. The slider is an obround (double D) and is brass running in a brass yoke. The prototype has many hundred hours on it. mostly running lightly loaded, and there is no appreciable wear in the yoke mechanics.

Gail in NM

 

[Anatol]

I like Scotch Yoke drives. They can be built very short giving a lower center of gravity which can be helpful for marine applications. If you make the valve gear drive a Scotch yoke also it gives you a true sine wave motion which makes timing very easy. Main down side is they don't look as elegant as conventional crosshead designs. No appreciable difference in efficiency.

I built my first one some 35 years ago and later sold it as a kit for about 15 years. Sold the kit business. but the kit is still in production. It was 1/2 bore X 5/8 inch stroke. The slider is an obround (double D) and is brass running in a brass yoke. The prototype has many hundred hours on it. mostly running lightly loaded, and there is no appreciable wear in the yoke mechanics.

Gail in NM

Interesting, Thankyou. How can I see the engine you describe?
I hadn't thought about driving a scotch yoke for valves, that's an interesting idea!

 

[jimsshop1]

I like Scotch Yoke drives. They can be built very short giving a lower center of gravity which can be helpful for marine applications. If you make the valve gear drive a Scotch yoke also it gives you a true sine wave motion which makes timing very easy. Main down side is they don't look as elegant as conventional crosshead designs. No appreciable difference in efficiency.

I built my first one some 35 years ago and later sold it as a kit for about 15 years. Sold the kit business. but the kit is still in production. It was 1/2 bore X 5/8 inch stroke. The slider is an obround (double D) and is brass running in a brass yoke. The prototype has many hundred hours on it. mostly running lightly loaded, and there is no appreciable wear in the yoke mechanics.

Gail in NM

Oops, I thought we were going to have discussion about finer Scottish Whisky there for a minute

 

[GailInNM]

The easiest way to see the engine is to go to:

http://grahamind.com/downloads.html

and down load both the brochure a manual for the SC1A. The manual contains a full set of drawings for the engine with enough details that an engine could be constructed from scratch. As the kit is pre-machined some detail dimensions may be missing, but they can be worked out if needed.

Gail in NM

 

[GailInNM]

The easiest way to see the engine is to go to:

http://grahamind.com/downloads.html

and down load both the brochure a manual for the SC1A. The manual contains a full set of drawings for the engine with enough details that an engine could be constructed from scratch. As the kit is pre-machined some detail dimensions may be missing, but they can be worked out if needed.

Gail in NM

 

[jimsshop1]

That's neat, now I understand its a slide crank, I've see those before. Thanks for posting. Glad to see some one active in the USA!

 

[Anatol]

The easiest way to see the engine is to go to:

http://grahamind.com/downloads.html

and down load both the brochure a manual for the SC1A. The manual contains a full set of drawings for the engine with enough details that an engine could be constructed from scratch. As the kit is pre-machined some detail dimensions may be missing, but they can be worked out if needed.

Gail in NM

Thanks Gail
in the hosting/software change - which I like - I lost my notifications, so I did not see this till now.
thanks again. The doc remarks on the higher frictional losses of the scotch yoke. Can this be reduced by using a roller or needle bearing on the pin (and a wider slot in the yoke?)

 

[bobden72]

Hi all
what are your opinions on the (efficiency, reliability) of the scotch yoke? It seems a clever way to avoid making cross slides. What are the pros and cons? Should it be made of cast iron for wearing? I guess the rod could have a sealed bearing on the end, running in the yoke?

One of the cons for this engine is that it is non reversing. It is timed to run in one direction, to make it run in the opposite direction it has to be re-timed.

 

[GailInNM]

A ball bearing could replace the slider , but in my opinion it would not decrease the friction enough to make it worth while. Consider that the horizontal velocity of the slider is highest when the piston is around top and bottom dead centers. Here the pressures on slider is at it's lowest point as there is no pressure being applied to the piston. At mid stroke, where the pressures on the slider are greatest , the slider horizontal velocity is lowest.

Contrast this with the more conventional crosshead with a trunk crosshead guide. Here the crosshead velocity is highest at mid stroke and the side load between them is also at it's highest point. So the difference between the Scotch yoke and the more conventional configuration is not that great.

For reversing, a slip eccentric can be easily fitted. They are not very elegant looking, but they work well. Nno single cylinder engine will self start reliably, so the flywheel must be given a partial turn to get them started. With a fixed non-reversing eccentric this needs to be a 1/4 to half a turn. With a slip eccentric this is the same if the engine is being started in the same direction as it was stopped. If desired to reverse the direction, it just a matter of turning the flywheel in the desired direction of rotation for 3/4 or a full turn as pressure is being applied. No big deal as I normally give the flywheel a flip with my finger that is adequate to turn the engine over that much anyway.

If a more elegant look is desired, a flat spur gear differential reversing gear can be fitted. They are not seen too often so they always seem to attract attention. Chedar models made a commercial twin cylinder marine engine that used this type of reversing gear. Still have to give the engine to start in most cases.

Bot of these reversing gears can be fitted int the SC1 without increasing the height, and both will work with the scotch yoke driven eccentric. Neither of these reversing gears will allow variable cutoff, in practice few of us ever run our model engine s notched up.

Gail in NM

 

[GailInNM]

A ball bearing could replace the slider , but in my opinion it would not decrease the friction enough to make it worth while. Consider that the horizontal velocity of the slider is highest when the piston is around top and bottom dead centers. Here the pressures on slider is at it's lowest point as there is no pressure being applied to the piston. At mid stroke, where the pressures on the slider are greatest , the slider horizontal velocity is lowest.

Contrast this with the more conventional crosshead with a trunk crosshead guide. Here the crosshead velocity is highest at mid stroke and the side load between them is also at it's highest point. So the difference between the Scotch yoke and the more conventional configuration is not that great.

For reversing, a slip eccentric can be easily fitted. They are not very elegant looking, but they work well. Nno single cylinder engine will self start reliably, so the flywheel must be given a partial turn to get them started. With a fixed non-reversing eccentric this needs to be a 1/4 to half a turn. With a slip eccentric this is the same if the engine is being started in the same direction as it was stopped. If desired to reverse the direction, it just a matter of turning the flywheel in the desired direction of rotation for 3/4 or a full turn as pressure is being applied. No big deal as I normally give the flywheel a flip with my finger that is adequate to turn the engine over that much anyway.

If a more elegant look is desired, a flat spur gear differential reversing gear can be fitted. They are not seen too often so they always seem to attract attention. Chedar models made a commercial twin cylinder marine engine that used this type of reversing gear. Still have to give the engine to start in most cases.

Bot of these reversing gears can be fitted int the SC1 without increasing the height, and both will work with the scotch yoke driven eccentric. Neither of these reversing gears will allow variable cutoff, in practice few of us ever run our model engine s notched up.

Gail in NM

 

[Anatol]

Hi Gail
I've been looking into scotch yokes. I see the attraction Reducing friction is key. I see some scotch yokes have a rectangular slot and a rectangular or square block pivoting on the crank pin. This adds one more bearing (friction) surface, but the sides of the block purely reciprocate. It seem like making such a block of a self-lubricating material would improve matters. What do you think?

 

[GailInNM]

Anatol.
Of course reducing friction is good on any engine.
For talking purposes I am going to use the SC1 as an example as you have the drawings available. Not suggesting that you build one. A little background on the SC1. It was not designed as a high efficiency engine. It was part of an experiment of what would happen if modern production techniques were used to produce a low cost entry level machined kit with reasonable performance and ease of assembly. Over the years the design has changed some from the drawings in that the castings for the base and flywheel were changed to machined bar stock.

To increase performance I would first add ball bearings for crank shaft bearings. At the time of design ball bearings were too expensive to add to the kit. Now suitable bearings are available for less than US$1.00 each.

Next I would change the slider to either a ball bearing (R133zz) or change the slider to acetal (Delrin). Current drawing use a steel slider in a brass yoke. The cooef of friction for steel on brass is about .35 and acetal on brass is about .15. I would choose acetal over PTFE (Teflon) because of of it's hardness and ease of machining. It is also more dimensional stable and does not creep under load as much as PTFE. This also reduce the side loads on the yoke guides. They are not that high but the yoke does try to cock in guides a little bit because the crankpin is not in line with the piston rod. I would line the guides with a lower friction material. I use a reinforced PTFE adhesive film for this sort of thing. It is used for lining the sides of conveyor belts to ease the flow of packages. I can end you some in an envelope (NC) if you need some to play with. I have 0.004 and 0.010 thick tape.

Timing can also be changed by changing valve dimensions and eccentric position depending on what your goals are. They will be different if you want the lowest unloaded RPM vs a working engine driving a load.

I will be leaving town for a steamup in Utah early next week Be gone for a week so will am not ignoring you if you want further discussion.

Best wishes,
Gail in NM

 

[Anatol]

Anatol.

To increase performance I would first add ball bearings for crank shaft bearings. At the time of design ball bearings were too expensive to add to the kit. Now suitable bearings are available for less than US$1.00 each.

Next I would change the slider to either a ball bearing (R133zz) or change the slider to acetal (Delrin).

I can end you some in an envelope (NC) if you need some to play with. I have 0.004 and 0.010 thick tape.

Best wishes,
Gail in NM

Thanks Gail.
Crankshaft bbs makes good sense.
Regarding slider I'm keen to play with some molybdenum disulphide impregnated nylon - this might be a good use for it (very slippery). I see moly filled UHMW PE is available, but I don't see moly filled acetal/delrin. I've also been thinking of casting and machining some graphite filled epoxy.

Thanks for the tip on the ptfe tape, sounds like an interesting product to have in the toolkit.

I was looking at George Carlson's double scotch. Making a 2 cyl single acting scotch yoke engine seems like an interesting idea because the second cylinder stabilizes the yoke. It is an extra cylinder though .
I was thinking about using the slotted crankshaft type rotary valve. It does add fairly long steam passages but it keeps the parts count way down
thanks again.

 

[GailInNM]

Anatol.
I have used some MD filled Nylon for various purposes in the past. The Coefficient of Friction of MD filled Nylon on metalsis about .3 to .3 depending on the compounding used. This is slightly higher than the .15 typical value for acetal. Not enough to worry about. The MD Nylon has better impact resistance but that would not be a factor for a slider.

I personally would use acetal because it is much easier to machine. To machine the MD Nylon cleanly the speeds and feeds are more critical and the tools must be very sharp. Acetal is on of the easiest to machine plastics. Any tool that works well on alum and is reasonably sharp will give a good finish using speeds and feeds of That Looks About Right.

I am not a big fan of slotted crankshaft valving. It is difficult to keep leakage from affecting low speed performance. Think lapping the sleeve and crankshaft to get a proper fit. Even then, any wear will again reduce the low speed operation. At higher RPMs it is not a problem. The same comments apply to piston valves in smaller scales where rings can not be fitted.

Gail in NM

 

[Anatol]

tool that works well on alum and is reasonably sharp will give a good finish using speeds and feeds of That Looks About Right.

I am not a big fan of slotted crankshaft valving. It is difficult to keep leakage from affecting low speed performance. Think lapping the sleeve and crankshaft to get a proper fit. Even then, any wear will again reduce the low speed operation. At higher RPMs it is not a problem. The same comments apply to piston valves in smaller scales where rings can not be fitted.

Gail in NM

thanks for your quick reply.

"that looks about right"?

Thanks for your advice about slotted crankshaft valving. I recognise the seal problem. But the idea is so attractive because its so simple - its the usual tradeoff. I *suppose" you could put tiny rings on? What do you think about labyrinth seals in such a case? (I'm pretty intrigued by the whole idea)