Peter Neill
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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
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