Engine is binding for some reason : 7BIM engine by PM Research.

I am building the 7BIM double piston engine by PM Research. The two cylinders are not yet connected with a manifold ; I am running each cylinder on air to test them. I can get them running nicely for about 15 minutes but then they start binding up...usually at BDC.

**LINK**

I think the clue lies in step 4 of the instructions :

''Adjust piston rods in crossheads until piston strokes are centred in cylinders. Torque screws evenly to prevent pistons from binding.''

How does the reciprocating motion of the piston cause a torsional force ? If I can understand what is happening then I can work out a way to prevent it.

NOTE : I have already built the single cylinder 3BIM but this must have been a fluke as it runs perfectly !

They are not talking of a torsional force, by "Torque" they mean tighten the screws evenly. If oine screw is tightened more than another it will pull the parts out of line or twist it.

If they are running well for 15mins and then binding up I would suggest its lubrication that may be an issue.

Brian, (you probably do), inject some oil in the inlet before the run, and every now and then.

Ian S C

It might be getting very cold if you are running it on high-pressure air for that long. This could cause binding where you wouldn't expect it.

Neil

That cold can also cause a build up of condensation, you can't compress water.

Ian S C

No, it is mechanical binding. When I release the nuts at either end of the piston rods then things start moving again. I do not understand this at all. There is enough lubrication.

I might try loosening the piston rod nuts and holding them in place with Loctite and see how this goes ie. I will not tighten the nuts against the crosshead and the piston. I will leave some play there.

NOTE : these piston rod nuts control the stroke of the piston. It is how the piston is centred in the cylinder.

Edited By Brian John on 03/09/2014 14:29:22

What are the cylinder and piston materials?

Neil

Sounds like there is a slight missalinement between the piston rod and the hole in the cross head, all is well when the nuts are loose as the piston rod can move slightly within the crosshead but once the nuts are tightend you loose that flexibility and it sticks.

Try turning the cross head top for bottom as the hole may be off height wise. while you are doing that make sure the crosshead can move freely between the top & bottom guides without the piston attached.

The problem with leaving the nuts slightly loose is the piston rod will start to fret in the crosshead and eventually become looser as things wear.

1. The cylinders are brass and the pistons are stainless steel. Nothing is running long enough to get hot ; the different expansion rates will not be the problem. The 3BIM engine runs well.

2. The crossheads move freely without the pistons attached.

3. I will try flipping the crossheads over tomorrow but as these are probably machined in bulk I do not think they will be out of alignment. At this stage I will try anything !

4. I will disconnect one piston/cylinder and just concentrate on getting the other side to run without any problems.

Edited By Brian John on 03/09/2014 18:11:24

> The cylinders are brass and the pistons are stainless steel. Nothing is running long enough to get hot ; the different expansion rates will not be the problem. The 3BIM engine runs well.

That's what I wondered. If it's getting very cold the brass will contract much more than stainless steel. If this is your second engine you may have made the pistons a better fit in the cylinder - at room temperature...

Neil

Neil, its a machined kit. Which is the likely reason for some bits being a bit sloppy and the others tight.

J

OIC

Neil

I have turned the crossheads over and it did not work ; the engine is still binding. I think the only thing left to try is the Loctite on the piston nut.

NOTE : I think I could disassemble and assemble this engine in the dark now !

Edited By Brian John on 04/09/2014 05:21:59

Imagine if the con rod ( piston to cross head).. is threaded slightly drunk...

Rotating the rod would alternatively make the assembly tight and loose...

A bit OT, but similar, I'm fiddling around with a design of a rhombic drive Stirling Engine, and when I fitted the gland for the displacer rod in the piston I found the it was not square, it screws into the piston crown with a 3/8" UNF thread, I eased off the thread, put on some Loctite, loosely tightened the assembly, and held it sqare until the glue set, seems ok so far.

Ian S C

UPDATE : using Loctite on the piston nut did not help but it does not hurt either. Now I will not have to worry about this nut coming loose. I found that the binding only occurred under pressure and when I disconnected the air hose and turn the flywheel by hand the engine gradually loosened up. This indicated a problem with my valve settings. I found that the valve on the RH cylinder was not centred so I unscrewed it from the eccentric one full turn and this seems to have solved the major problem.

1. The RH cylinder is still giving a few problems : it requires about twice as much pressure (about 5 PSI) as the LH cylinder and even then it runs much slower. There is also a knock which I cannot locate. Any suggestions there ?

2. I also removed the gland packing as a possible source of friction and I have found that it makes no difference to the running of the engine. What is the point of it ?

Edited By Brian John on 06/09/2014 05:29:51

Edited By Brian John on 06/09/2014 05:31:28

> 1. The RH cylinder is still giving a few problems : it requires about twice as much pressure (about 5 PSI) as the LH cylinder and even then it runs much slower. There is also a knock which I cannot locate. Any suggestions there ?

It possibly just need a bit more running in. 5 psi isn't very much.

Neil

I am still having problems with the RH cylinder : it runs for about 10 minutes then gets slower and slower and stops. Yet both the left and right side have been set up the same in terms of valve and piston settings. I am really puzzled at this.

Edited By Brian John on 08/09/2014 06:50:39

UPDATE : The piston and the rod are still turning hence everything is getting out of synch. I will have to tighten the locking nut on the piston rod against the cross head to stop this happening. I did have it tightened at the other end of the thread to make it easier to adjust the length of the piston rod. (Two nuts and a slightly longer thread would be more useful here.) I am surprised that there is so much movement. The single cylinder 3BIM does not have this problem.

Try again tomorrow !

Hi Brian,

Try putting 'magic marker' on the rubbing surfaces of the piston, crosshead and piston rod.

This will rub off most rapidly at the points of greatest contact and may help you track down exactly where things are binding.

Neil