Piston/Bore Clearances ?

[Peter Neill]

This is a slightly oddball question, but to be honest I can't think of a better place to ask the question.
First a little background information.
I have a little development project to look at to help out a customer of mine. Nothing to do with my normal injection moulding/toolmaking business, but they're short of engineering support on the ground and I was asked to help.

The problem is with a tiny 12V air compressor - like the ones you plug into the car cigar lighter that takes 10 minutes to inflate a flat tyre. These are used on one of their instruments, but fail with alarming regularity, possibly due to the duty cycle. A small 12v motor powers a very simple crank with a cast aluminium piston, using a single rubber piston ring, inside a simple metal sleeve for the cylinder. I don't know if the sleeve is steel or cast iron, but it just looks like a bit of bog-standard tube that has been cut to length and sandwiched between 2 end plates.

There are 2 problems. First, the rubber piston ring (more like a hydraulic piston seal) destroys itself with chunks breaking off or wearing and producing lots of small particle debris, so the compressor fails to make any pressure.
Second, the gudgeon pin (wrist pin for you guys on the other side of the pond) is obviously not a precision fit, and works loose from the piston until it digs into the bore and wears a nice big slot in the side of it.
These things are obviously made cheaply for very occasional use pumping a tyre up, so with the extended duty cycles they fail. The duty cycle is still only 5 minutes at a time, but needs to operate a few thousand cycles. As yet, they have not found an alternative supply source for these items.

My thoughts are that these may possibly be improved by using better/different materials and better fits by machining up replacement items for some of the original parts.
Volume requirement is only around 100-200 units per year.

The cylinder sleeve is 19.25 OD and 16.00mm bore x 25mm long. It's a plain sleeve with no fins/heatsinks. The diecast piston is 15.4mm diameter and 9.5mm length, and the rubber seal is 16.5mm diameter. The gudgeon pins are 2.5mm diameter, but the pin hole in the piston is 2.6mm diameter, and has just been counter sunk with a 4.5mm drill to provide some clearance for the pin ends to sit below the surface. The piston is a very simple design and is not 'finished' at all, but just as cast really.
As you can see there is over 0.5mm clearance between the piston and the bore, so when the ring/seal wears a bit this can tilt over and may contribute towards both the gudgeon pin coming out and also probably accelerate the wear on the ring/seal.

I thought of making both a new cylinder and a new piston with much closer fits, and using a UK sourced hydraulic seal ring.
What sort of clearance should I aim for on the cylinder and piston, and what materials should I use? I have both leaded bronze and phospher bronze and steel (multiple grades) available, and I can also get the piston and/or cylinder coated with very hard, very low friction coatings that are only microns thick, if this would help.
I have no idea what the running speed of this is, but the small 12V DC motor is ca. 35mm dia and 50mm long, and is geared down by around 5:1 to the crank assembly.

Any and all advice welcome. Sorry it's a bit long winded!

Peter

 

[Bluechip]

Hi Peter

Yes, I have one in my shed. The words 'primitive' and 'rustic' spring to mind.

Can I ask why a piston pump??

I had a diaphragm pump on my airbrush, gave circa 15 - 20 psi, and enough to run a rather small gravity feed gun.

No pistons, o-rings, piston blow-by, sieze ups etc. etc.

Mine was ex-medical gear, far from new when I got it, ran for probably 200+ hrs in total. Still worked when I gave it away.

From memory, dia. was 3 - 3.5", operated by a cam maybe .375 - .5" stroke.

Original motor was 28v DC, I ran with a 240v 1/10 HP [ 75w ] induction motor 1425 rpm

Original motor was knacked, which is I why I got it. Replacement unit was in one bit, ie. motor/pump unit.

Dave

 

[rickharris]

If you go for the diaphram solution have a look at a mechanical car fuel pump = all you need it to provide a one way valve and a drive for the cam.

 

[Peter Neill]

Unfortunately replacing it with a different pump is not an option.

These pumps are fitted inside the instrument case (which is fitted inside an aluminium briefcase) with an allowable footprint including the 12V DC motor, of approx. 3" x 3" x 3".
Additionally, they have to be retrofitted & replaced to instruments already out in the field. The 12V power supply is taken directly from a feed off a circuit board, and there is no more power available.
Pressure required from these for the application is 60psi.

I don't know why they originally chose a piston pump, but it is at the moment what they are stuck with.

Compound Driver has sent me a PM with a few useful ideas (thanks very much for this), and I've just had another thought as well. What about the possibility of making a new piston from PTFE, with a slight outboard chamfered edge at the top, to compress under fitting & load to form an integral seal? Or perhaps a very close fitting leaded bronze/gunmetal piston without a piston ring?
The existing seal is a a lip seal, more like a hydraulic/pneumatic seal than an 'O' ring.

I've never built an engine before, and I know isn't exactly one either, but there must be similar issues that you chaps have come across.


Peter

 

[macona]

Straight teflon is too soft. Glass reinforced teflon would be better but I would use delrin, nylon, or nylatron.

There are other small air pumps in that size that would be a lot better than that one. Designed for continuous use too. I have one. If I can find it I will get the manufacturer off it.

 

[mrehmus]

There are simple solutions to your problem I think. These suggestions are starting points. All the specs are the same as would be used in a hit and miss engine except the O-Ring material would change to an off-the-shelf Buna-N or similar material.

1. The O-Ring OD should just match the cylinder ID.
2. Cylinder ID finish should be as polished as possible, Chrome would be good.
3. Piston to cylinder clearance of 0.002" will work OK.
4. A reinforced Teflon ring should work since you don't want lube in the cylinder.
5. O-Ring clearance should be 0.005" on each of the 3 sides of the piston ring groove.
6. Teflon pads on both sides of the wrist pin will keep the pin in place and not damage the cylinder.

This combo will exhibit no compression when you turn it over slowly but will have good compression when spun rapidly. This characteristic will allow the electric motor to start up with a low load.

O-Ring life in hit and miss engines using the above specs is measured in years when they run constantly at shows for 4-8 hours per day.

 

[Peter Neill]

Many thanks Mrehmus, that is just the sort of information I was hoping for.

Peter

 

[crankshafter]

Peter.
If you go for o-ring, go for a Viton-ring, will last for years
CS

 

[Cedge]

Why does this one thread strike me as simply being a request for free R&D for a profit making endeavor? Moderators?

Steve

 

[Peter Neill]

Why does this one thread strike me as simply being a request for free R&D for a profit making endeavor? Moderators?

Steve

Perhaps because that is exactly what it is Cedge, so where is the problem with that? Are you suggesting that it should be locked or deleted because I might make a few bob out of the job?
As I said earlier, I can’t think of a better place to ask this sort of question as it is completely out of my sphere of experience, and thanks to the posters here I have obtained some good advice.

Whilst I am a new member here, I have been around on UK and other international HSM and model engineering forums for years, and in turn I have helped many people out with free advice, free material, free tools, transport around the UK and even sub-contract machining work – which I don’t do – which has earned one model engineer in particular several thousand £’s.
There are several posters here whom I am sure would be happy to vouch for the above.

As such I think that your allusion is perhaps a little bit uncharitable.


Peter

 

[Cedge]

Peter
At least you are honest enough to admit your motivation. I find it a bit tawdry, lacking in ethics and more than a little parasitic, especially considering the board is clearly a hobby forum. So yes... I do find it very offensive....thanks for asking. Might I suggest paying for your research like everyone else in the real world?

Steve

 

[chip_monk]

Hi All,

I have been prompted out of lurk mode to help defend Peter, I have to declare an interest as Peter has helped me personally to equip and use my workshop but please consider the following points.

1. Many of the members of this forum are openly engaged in engineering for profit. Most of them give freely and generously of their time and expertise. Is it not reasonable to give a little back if possible.

2. Peter is far too honest to use devious methods but some are not, I would far rather have the choice whether to contribute or not knowing that the information may have commercial value. Lets not drive the process underground or drive the professionals away from the forum.

3. The whole purpose of the forum is to spread knowledge so that we all profit, be it in coin, kudos, time, buying knowledge, safety or simply more enjoyment from the hobby. Can we avoid adding to the web of lines we have to navigate in order to enjoy an honest, open, informal and informative discussion on workshop topics.

Thanks to all who have helped me, mostly unknowingly, with this fascinating hobby.

David

 

[wareagle]

Gentlemen, let's not turn this into a counter productive thread.

 

[Cedge]

David
I'll acknowledge your points, but they change nothing in my mind. I'll stand by my point. I've not besmirched Peter's contributions, but I am very much questioning the propriety of his actions in this instance. The members of this board have always been very generous with information, but mining them for free R&D is both unfair and.... as I said before... IMHO lacking ethics. I seriously doubt anyone here will be seeing royalty checks for the profitable use of their ideas.

Steve

 

[Mcgyver]

Peter
At least you are honest enough to admit your motivation. I find it a bit tawdry, lacking in ethics and more than a little parasitic, especially considering the board is clearly a hobby forum. So yes... I do find it very offensive....thanks for asking.

if you do, walk on by....if you don't help him out....if the everyone agrees with you he won't get help, if they don't he will, either way I can't see why its worth making a fuss over

 

[Peter Neill]

It is a bit of a Storm in a Teacup isn’t it? Lot of fuss about nothing really.

I’m grateful to those who have pm’d me and also those who posted their advice here, it’s all useful and I’ll put it into practice next week.
If the thread doesn’t end up locked I’ll post the details of the parts once made, and the results of the testing when they are available.

To clear up a possible misconception let me add this.
I’m making a couple (3) of prototype piston/cylinder assemblies. If they work, I’ll supply the specs they were made to, and then that is where my involvement ends. I’m not going into production on them, and I certainly won’t be getting any royalties. These are nothing to do with my normal business of Injection Moulding, but they are helping out a good customer of mine. I’ve quoted them one days work to make the parts, and they’ll be made on my Myford, in my home (garage) workshop, with material I already have.

Steve, everyone has an opinion (there’s a saying about that somewhere...) and of course you’re entitled to air yours, despite the paucity of thought.
The wonderful thing about forums is that people can ask for information on appropriate subjects where they are ignorant, as I freely admit that I am concerning model engine clearances.
Whether a professional or a hobbyist is asking the question should make no difference.
I’m sure that if I had phoned up Ricardo Engineering to ask the same question they would have laughed me off the phone.

Steve, I’m not interested in a little flame war or a slanging match, and I think this is a pleasant forum that is worth better than some of the comments made, so this will be my last word on the subject other than further details of the project.

Peter

 

[Cedge]

Peter
You're not the first to mine ideas from this forum for commercial gain and those prior efforts didn't get a free pass either. The only paucity of anything that I can see is as stated above... a lack of ethical behavior from those who play that game. I'll stand by what I've said.

Just as it was when you were a student.... It's your assignment.... kindly do your own homework for your own grade. When done, you're still invited to join the rest of us for more recreational endeavors.

Steve

 

[Maryak]

Please Guys,

Lets call it a draw shake hands and move on.

Thanking you both

Regards
Bob

 

[tel]

I second the motion Bob, the issue is dead in the water. The opinions have been stated, anything else is just Ditto reapeato ad nauseum. Let it be.

 

[compound driver 2]

Peter
Ask away as far as im concerned your well with in your rights to look for help on here. This is a free forum for friends and engineers. Go for it.

On the point of Cedge not wanting comercial interests, how about the web site Cedge has pasted on every post he puts up. I think youl find thats got a comercial bent to it. So maybe practice what you preach springs to mind.
Live and let live its a free world.