Steam engine piston

[deeferdog]

Hi everyone,I am building a steam engine from a set of plans I purchased. They are for a set of castings, however I am attempting to build it from scratch. I have hit a snag with the piston as I cannot understand what the drawing means.Any help would sure be appreciated. I am only a novice so explanations limited to one syllable would be appreciated. I have attached pics of plans. Thanks again.

 

[Cogsy]

I've never seen a piston like that (I'm not a steam guy) but it looks to be 2 pieces, held together with the 3 x 1/8 bolts, that hold a third piece (likely a ring or sealing material) captive between them. The centre hole is also tapered by the looks of it. Personally I like a little more detail in my plans but I normally build from scratch. I guess if you had the castings to refer to it might be a fairly simple process of mating them together.

 

[Mike1]

Your drawing shows a built up piston with the two rings sandwiched between the top and bottom of the piston, if it was me I would make the piston solid and cut grooves for rings or graphite packing.

Mike.

 

[gbritnell]

The drawing shows a taper on the shaft that the piston is located and held on with. The problem I see with this is 1. The taper is backwards and 2. Even with a taper there should be a nut to make sure the piston doesn't come loose.
The piston is made in 2 pieces to allow the piston rings to be fitted. Maybe not a bad idea in full sized engine practice but not normal for model engine work. I also noticed the filler block bolted to the head. First of all this could be machined onto the head and secondly it leaves too much space for the steam to fill when the piston is at the end of it's stroke. Normally the head just has a relieved area where the port is to allow the flow of steam.
gbritnell

 

[Jasonb]

The two part pistons do save on rocipricating weight and were common on full size as well as ones that were cast hollow. There are a number of model engines that still use two part pistons thought a solid aluminium one could be used without altering the mass and need to adjust the crankshaft counter balance weights.

You also have an eliptical hole in the piston ring and a spring in one of the lugs on the main piston half to help keep it in even contact.

As George says the taper is usually the other way round but it is possible the piston is held onto the rod with a tapered cone that screws on, can't really tell without seeing the drawing for those bits

 

[deeferdog]

Thanks to everyone who has helped with this, I can now understand how the piston is constructed. I think I'll just do a solid one from aluminium and cut he grooves for the rings as suggested. As always, one explanation leads to another question: What is a filler block and what is its purpose? These "plans" are more an exercise in detective work than machining but I am enjoying the challenge.

Regards,
Deeferdog

 

[Jasonb]

Again without seeing the whole drawing it is a bit hard to say. A filler block can be used to reduce the "dead space" between piston and head as George call it, I would say the cylinder cover. Its the "top hat" shaped block on your drawing retained by teh single central stud and nut.

Though looking at the drawing it looks like you have a vertical cylinder and that could be bolted straight down onto the engines bedplate in which case it is not practical to have an integral filler block so the separate one like you have is the better option.

J

PS out of interest what is the engine you are building?

 

[deeferdog]

The engine is a Bolton 12 Beam engine. It has been built by others in this forum I think. I brought the plans from E J Winter but decided against buying the castings as they are a lot of money and I doubted my ability to complete the project. My lathe and mill are both Chinese and hobby size but I really enjoy using them and because I hated the thought of wasting the $75 I paid for the plans, I decided to have a go at building from scratch. On and off for about six months now and I have made a bit of progress. I wish the plans were better, there's a lot left out, I assume if one had the castings then things would be more obvious. The measurements are all in fractional inches, how we ever worked with that! I convert to metric and decimal inches and balance the result with the stock I have lying around. Some of the things, like the piston had me completely foxed so I really am grateful for your assistance. I have added a pic of the facing page of the plans.

Regards,
Deeferdog

 

[Jasonb]

The separate filler block make sense now as it would be almost impossible to turn it to fit the cylinder if it were integral to the base casting, could have been done with a boring head but not an easy setup on a lathe as most did not have mills when the engine was introduced.

As for fractions you need to remember that at the time this model was designed a steel rule and friction leg callipers where what was being used to measure things in the home workshop.

Should all be possible from fabrication or cut from solid, maybe use an off the shelf flywheel casting. I've just done a similar cylinder with the decorative bands around it by fabrication but for a horizontal engiene.

J

 

[kwoodhands]

Hi everyone,I am building a steam engine from a set of plans I purchased. They are for a set of castings, however I am attempting to build it from scratch. I have hit a snag with the piston as I cannot understand what the drawing means.Any help would sure be appreciated. I am only a novice so explanations limited to one syllable would be appreciated. I have attached pics of plans. Thanks again.

I understand the drawing but it makes a simple piston a PITA. I would turn the piston from brass or bronze and cut a groove in the center for a Viton ring.
I did this today, the piston is .874 and the groove is .096 wide and the same deep. You can fasten the piston to the rod several different ways, all easier than a tapered end. I threaded the rod end 12/24 x 5/16" long and tapped the piston. Loctite , soft solder or pinning the piston to the rod will do. I used red Loctite myself. Cut a slot for a screwdriver in the piston end to aid adjustment when the head is removed.
mike