Grub screws holding piston rods into crosshead

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Grub screws holding piston rods into crosshead

Post  ChriX on Sun 10 Jun 2018, 1:14 pm

On my engine the grub screws holding the piston rods steady in the crosshead are always coming loose which allows a very slight amount of play and hence starts to make a knocking noise.

Has anyone else had this issue? I have not tried Loctite on them yet as I fear I would never get them out again; they are so small and these hex head grub screws have a tendency to round off in my experience.

Thoughts, anyone? Smile
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Re: Grub screws holding piston rods into crosshead

Post  Mark the spark on Sun 10 Jun 2018, 6:04 pm

Hi Chris
Do you enough room to put a second grub screw in to lock the first?
Mark

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Re: Grub screws holding piston rods into crosshead

Post  Jim the Shovel on Sun 10 Jun 2018, 10:10 pm

Hi Chris

I was looking at that the other day. I don't like these small grub screws. They are too soft for starters.
I was thinking of increasing them to M5 and using dog point grub screws.
Once the crosshead as be re-tapped M5 the pistons and rods are refitted and set up.
Then I was thinking of possibly taking a M5 hex bolt and bore it to the diameter of the dog point.
This will be used as jig for drilling the piston rod to suit the dog point.

What's your thoughts. May be a bit over the top but at least there's no fear of the piston rod turning.


Regards

Jim
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Re: Grub screws holding piston rods into crosshead

Post  Alan Grant on Sun 10 Jun 2018, 11:22 pm

I have the same problems Chris, the grub screw came out completely at the last rally. I have tried Loctite again but it doesn't appear to be a good solution. At the moment i'm just checking it regularly when in steam and carrying spare grub screws!

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Re: Grub screws holding piston rods into crosshead

Post  Steve Traill on Mon 11 Jun 2018, 9:32 am

Go for stainless steel grub screws and fit two short ones. Locktiting grubscrews can be a problem when removing them especially with cheap allen keys as they round off. I found it was worth investing in a decent set of allen keys.
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Re: Grub screws holding piston rods into crosshead

Post  Jim the Shovel on Mon 11 Jun 2018, 10:41 am

Chris

Just thought of another answer. This is a bit simplier.
Put in a longer grub screw in, say 10mm and do up a locknut on top.
That should take care of the vibration.
This could still be done with the dog point I suggested earlier. Which
should now cover all bases.

Rewgards

Jim
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Re: Grub screws holding piston rods into crosshead

Post  ChriX on Mon 11 Jun 2018, 11:27 am

Thanks for the suggestions! I might have some extra short ones around to try putting one on top of the other but if not Jim your suggestion of a longer one with a nut sounds good too.

To the shed!
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Re: Grub screws holding piston rods into crosshead

Post  Jim the Shovel on Mon 11 Jun 2018, 2:20 pm

Hi Chris

To be honest I think putting two small ones in is a bit risky (sorry Steve)
If the head of the first gets damaged by the second there could be a problem getting if out.
Also definitely don't use loctite as you'll have no chance.


Regards

Jim


Last edited by Jim the Shovel on Mon 11 Jun 2018, 10:52 pm; edited 1 time in total
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Re: Grub screws holding piston rods into crosshead

Post  sonick45 on Mon 11 Jun 2018, 4:08 pm

suggestion.

put a little steam seal graphite paste down the hole after you have tightened of the grub. The heat from the engine will cure the paste solid but if you need to remove the grub you can carefully dig the steam seal out. it will give enough resistance to stop the grub coming loose but not to much that you cant remove it.
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Re: Grub screws holding piston rods into crosshead

Post  ChriX on Fri 22 Jun 2018, 8:27 pm

OK so I ended up using a normal hex headed bolt with a locking nut on the right hand side, and two grub screws, the outer with a very thin locking nut on the left hand side. I was planning to just use the bolt and locking nut on both sides but clearance is restricted on the left due to proximity with the oiler. We shall see what works.

Thanks to all for the suggestions!
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Re: Grub screws holding piston rods into crosshead

Post  Jim the Shovel on Sat 23 Jun 2018, 11:33 am

Hi Chris

I re-tapped the cross head M5 last night and drilled 3.5mm through a M5 bolt last night in readiness.
The M5 x 10mm dog point grub screws arrived this morning so am now ready for the modification.
My engine is not up and running as yet but hope to run on air this week.
Will keep you posted with how I get on. The M5 grub screw certainly looks a lot better.

Jim
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Re: Grub screws holding piston rods into crosshead

Post  Jim the Shovel on Fri 03 Aug 2018, 10:30 pm

I've finally done it. Had previously increased the thread size to M5.
Now that I'm satisfied with the piston rod set up it's time to drill for the dog point grub screw.
Had to make another jig for drilling this time using a cap head. Better grade bolt.

With 3.5mm drill, drilled through the jig and into the piston rod the length of the dog point.


The M5 x 10mm dog point is just long enough to add a half nut so it can be locked in.


There we go all done and dusted. No Loctite, No load on the grub screw and piston rod locked in position.
Best bit is it is now an easy matter to pick up same hole on re-build.
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Re: Grub screws holding piston rods into crosshead

Post  ChriX on Thu 09 Aug 2018, 11:59 am

Nice work Jim!
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