Alfa selespeed vertical plunger/piston

[markherbert]

Hello. First post here, long time "lurker"
I have a Selespeed "robot" here leaking and I believe the problem to be the rust on the vertical plunger/piston.
The geometry is quite simple, and I could make a technical drawing no problem.
I've also considered seeing if the shaft could be repaired however I think it would cost more than having one made, but correct me if I'm wrong. The shaft is 11mm diameter and about 40mm long
I'd like someone to machine a batch of them, my questions are as follows.
Material:
The end of this shaft is essentially open to the elements. The original seems be steel with plating (chrome/nickel??)
Would it be possible to make this shaft out of stainless, would a good enough surface finish be possible? Just thinking that would be simpler than having a shaft made undersized then chroming and polishing.
Cost:
If I was going with stainless, and wanted say 5 of these made, what sort of cost could I be looking at? Would be quite tight tolerances due to the shaft size and also surface finish would be important (especially if not plating)

And finally does anyone have any recommendations for a company or individual to make some of these for me

 

[markherbert]

Additionally I believe the shaft to be hardened after the rough machining was done?

 

[Kram]

The shaft is 11mm diameter and about 40mm long

It looks to be approximately 10 times the diameter so about 110mm long if its 11mm dia.

If the shaft is damaged, what of the components it runs in?

 

[m_c]

Although fairly simple geometry, that looks like a ground finish, which is typical for the tight tolerances required for hydraulic valves.

 

[markherbert]

It looks to be approximately 10 times the diameter so about 110mm long if its 11mm dia.

If the shaft is damaged, what of the components it runs in?

Apologies must have made a typo

The shaft doesn't touch any metal, it runs on PTFE scrapers with o rings behind for the sealing and then in addition there are some PTFE liners in the bore it sits in.

Although fairly simple geometry, that looks like a ground finish, which is typical for the tight tolerances required for hydraulic valves.

Do you think grinding would be required for small batches? From my limited knowledge I believe the tolerance could be achieved on a lathe however grinding would probably be faster for a large production?

 

[Kram]

Lathe wont hit the surface roughness just from cutting however with extra step of burnishing and polishing it should be acceptable. If it didnt have the larger steps in the middle, it may have been possible to use an off the shelf ground polished stainless bar/dowel pin as the start material.

Looked up stainless it would appear 410,420,440 are the quench hardenable grades.

 

[bricol]

Actual diameter tolerance can probably be reasonably large - maybe up to h11. Measuring the bore and the piston accurately will give you a clue.

But the grinding will be required to get the surface finish for the seals and o-rings. Somewhere in the 0.1 to 0.4 um range - which can be obtained by turning, but a lot more reliably from grinding. If plating, it will need polishing before and after to get the finish - hard chrome gives a good low friction surface for the seals to work on, which also allowing oil retention in the micro-cracks formed - without the oil retention, the seals will fail quicker.

It will work without the plating - just not for so long as the original.

 

[Agroshield]

Is 11mm a standard hydraulic or pneumatic cylinder rod size?

Imagine a standard hydraulic rod, with piston and securing nut. You reproduce that but the nut is the right hand end of the shaft pictured.

Make the left hand end with a reduced diameter where the double ring is and some male threads. Bore a register into the right hand end and female thread it. It would fit the left hand piece in the same way as a threaded-on lathe chuck, the register making sure the two parts were concentric and the loctited threads stopping it falling apart.

Before you screw the two parts together, put on the middle bit, made oversize. Once the loctite has set, turn the middle bit (which does not appear to touch anything, only acting as ashoulder to hold the O rings) to diameter and concentric with the chromed rod.

That way, you do not need to grind anything and are using bought-in material where the hardness and surface finish is needed.

 

[markherbert]

Is 11mm a standard hydraulic or pneumatic cylinder rod size?

Imagine a standard hydraulic rod, with piston and securing nut. You reproduce that but the nut is the right hand end of the shaft pictured.

Make the left hand end with a reduced diameter where the double ring is and some male threads. Bore a register into the right hand end and female thread it. It would fit the left hand piece in the same way as a threaded-on lathe chuck, the register making sure the two parts were concentric and the loctited threads stopping it falling apart.

Before you screw the two parts together, put on the middle bit, made oversize. Once the loctite has set, turn the middle bit (which does not appear to touch anything, only acting as ashoulder to hold the O rings) to diameter and concentric with the chromed rod.

That way, you do not need to grind anything and are using bought-in material where the hardness and surface finish is needed.

View attachment 464694

Neat idea, but I bet it would be hard in practice to get the join between the two halves perfectly straight. Since there wouldn't be turning of those portions after the fact, only the central piece. Maybe I'm wrong on this. I've phoned a supplier and they're going to see if 11mm hard chromed rod is something they stock. I think the tolerance is iso f7. I will have a measure today the rod and the bore and try and work out the tolerances on the existing part. Starting from a hard chromed rod already to spec would really be preferable cost wise I think

 

[markherbert]

Actual diameter tolerance can probably be reasonably large - maybe up to h11. Measuring the bore and the piston accurately will give you a clue.

But the grinding will be required to get the surface finish for the seals and o-rings. Somewhere in the 0.1 to 0.4 um range - which can be obtained by turning, but a lot more reliably from grinding. If plating, it will need polishing before and after to get the finish - hard chrome gives a good low friction surface for the seals to work on, which also allowing oil retention in the micro-cracks formed - without the oil retention, the seals will fail quicker.

It will work without the plating - just not for so long as the original.

Thanks for the reply. Will measure the tolerance today. On second thoughts I'm not sure that the original is plated at all. Trying to find out a way to test this

 

[Agroshield]

Neat idea, but I bet it would be hard in practice to get the join between the two halves perfectly straight.

Just guessing at some numbers, I was thinking M5 female thread, 6mm dia. reamed hole above that thread in the RH piece and then 6mm dia. plain portion on the LH piece (with M5 male thread). The reamed hole could not be anything but concentric with the outside of the RH piece. The plain portion could not be anything but concentric with the OD of the LH piece.

In my drawing, maybe the medium diameter of the LH piece needs to be drawn longer and the smallest diameter there (representing the male thread) needs to be shorter. It is the medium diameter piece that assures concentricity.

The threads need to be slightly loose so they do not try to fight with the fit between the reamed hole and the male spigot.

 

[markherbert]

Taking many readings with a micrometer I get results between 10.980 and 10.985

 

[markherbert]

And not knowing the design specification doesn't help. One assumes it's meant to be 11mm but perhaps not

 

[markherbert]

I have another shaft here which measured 10.984-10.986mm

 

[bricol]

Shafts tend to be toleranced undersize - you figures fit within the g7 limits I'd probably slap on something similar.

 

[markherbert]

Shafts tend to be toleranced undersize - you figures fit within the g7 limits I'd probably slap on something similar.

Seems around the tolerances of hard chromed rod that I find online that are either 10 or 12mm dia
But unfortunately I can find nothing (other than on AliExpress with not many specifications) for 11mm

 

[Kram]

Starting from a hard chromed rod already to spec would really be preferable cost wise I think

The added complexity may make that more expensive than just turning or grinding, VS just using stainless to begin with.

However if you want the part to be chromed anyway, yes it may be best.

A futher option is to turn the part undersize in stainless and then bond a chrome tube sleeve as the wear surface, perhaps even a PTFE or POM sleeve would also be sufficient.

 

[bricol]

Seems around the tolerances of hard chromed rod that I find online that are either 10 or 12mm dia
But unfortunately I can find nothing (other than on AliExpress with not many specifications) for 11mm

11mm isn't exactly a standard size in bar stock, full stop.

 

[bricol]

The added complexity may make that more expensive than just turning or grinding, VS just using stainless to begin with.

However if you want the part to be chromed anyway, yes it may be best.

A futher option is to turn the part undersize in stainless and then bond a chrome tube sleeve as the wear surface, perhaps even a PTFE or POM sleeve would also be sufficient.

Now that triggered a thought . . but annoyingly, SKF Speedisleeves start at 11.99mm diameter . . .

 

[markherbert]

I think perhaps the way to go would be to use stainless, and have it ground. But how is it ground with the central raised portion in the way, do they have to grind one end at a time? Also I'm not convinced the original is plated at all. Still yet to find a way to test this