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Hydraulic Cylinder Design and Shortening

Doug W

Hot Rolled
Joined
Sep 22, 2003
Location
Pacific NW
I have a large 7" bore, welded body hydraulic cylinder with a long stroke that I want to make into 2 shorter stroke cylinders.
Doing so all that I would have to make is a end cap, gland nut, nut and piston and I have the existing ones to copy.

I have experience shortening cylinders, welding end caps etc,, not redesigning them.

The questions.

1. Does anyone know how to calculate the allowable pressure with the given wall thickness AND changing from a rear cap end clevis to a rod end flange mount?
I believe that change would add an axial load to the cylinder walls and end cap.
The cylinder has no manufacturer markings, rating, etc.
Upon disassembly the hydraulic parts supplier can give me an idea what the existing seals were rated for.

2. The flange would be a thick plate with a hole, slid over the cylinder tube and welded in place.
Is this done before the cylinder's internal thread is cut?
Does the weld shrinkage require honing afterward? Or is there some way to offset the shrinkage by for example having the plate hole oversize?

Answers to these specific questions will be much appreciated, tangents and diversions not so much.

Thanks!
 

Phil in Montana

Stainless
Joined
Jul 31, 2007
Location
Missoula Mt
You do all machine work after you are done with the all the welding, shorter is better for higher pressure, at 8inch bore the wall will be quite heavy, you will need to find a on line engineering program for the burst pressure and de rate from there, I know a 8 inch bore with a 1 inch wall is used for 4000psi...Phil
 

Strostkovy

Stainless
Joined
Oct 29, 2017
I think it would be easier to cut it apart, measure everything, and machine new duplicates and only salvage the honed tube.

I don't know exactly where the cutoff is, but large diameter cylinders aren't limited by cylinder strength, they are limited by cylinder stretch. A 1" diameter cylinder and 8" diameter cylinder have the same tolerance on bore size for the seals to work at a given pressure. But an 8" bore will stretch a lot more with pressure and so you run the risk of extruding the seals through the larger gap at high pressures.

If the cylinder is double acting and rated to the same pressure each way then it shouldn't matter which end the cylinder is mounted from. Even on a single acting cylinder I would think it would only increase axial tension, which a cylinder should be able to withstand at full pressure without relying on a load to keep it compressed. But I'm no expert here and I didn't think about it for very long.

I have seen people not worry about remachining bores of cylinders after shortening them by just having the welded area far enough away from the seal's maximum travel that any introduced distortion wouldn't matter.
 

idacal

Hot Rolled
Joined
Aug 9, 2011
Location
new plymouth id
I have welded after machining then spent 2 days trying to get the bore cleaned back up and the thread straightened out I ended up up using one of those lisle hones with the screw adjust to clean the bore back up
 

gbent

Diamond
Joined
Mar 14, 2005
Location
Kansas
You are welding a new butt end on the cylinder? When you make the butt, you make it so there is a spud that goes inside the tube. Then when you weld the butt end there is no shrinkage.
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
Ive made hundreds of cylinders and shortened old ones .....even welded a few in the travel area for a 'bodge" repair for the tractor dealer next door......Ive never seen a tractor/dozer /excavator /crane cylinder with anything like 1" wall,unless it was aluminium.......I used to make presses as a sideline using 7"and 8" dia offcuts ,and never used anything thicker than 1/4" wall approx.Working pressure was 3500-4000psi.To keep prices down ,I used ordinary hose ,and ocassionally someone would say their hose had burst when they were pressing a extra tough job......With a manual press,I would proportion the pump lever so at max WP ,a 200lb mechanic would lift himself off the ground....Heavier operator obviously could achieve more pressure......there were no reliefs,unless an electric or air pump was used..
 

Doug W

Hot Rolled
Joined
Sep 22, 2003
Location
Pacific NW
You are welding a new butt end on the cylinder? When you make the butt, you make it so there is a spud that goes inside the tube. Then when you weld the butt end there is no shrinkage.

Other end,
"changing from a rear cap end clevis to a rod end flange mount"
 

Doug W

Hot Rolled
Joined
Sep 22, 2003
Location
Pacific NW
Ive made hundreds of cylinders and shortened old ones .....even welded a few in the travel area for a 'bodge" repair for the tractor dealer next door......Ive never seen a tractor/dozer /excavator /crane cylinder with anything like 1" wall,unless it was aluminium.......I used to make presses as a sideline using 7"and 8" dia offcuts ,and never used anything thicker than 1/4" wall approx.Working pressure was 3500-4000psi.To keep prices down ,I used ordinary hose ,and ocassionally someone would say their hose had burst when they were pressing a extra tough job......With a manual press,I would proportion the pump lever so at max WP ,a 200lb mechanic would lift himself off the ground....Heavier operator obviously could achieve more pressure......there were no reliefs,unless an electric or air pump was used..

I don't know what the bore is or the wall thickness till I cut it open but I suspect at least 3/8" thick.
I also suspect changing the support from end cap clevis to rod end flange doesn't significantly change the tube stresses.

I just want to make sure.

Womack has a chart for cylinder wall thickness using the lowest grade of steel cylinder tubing 1020, with a safety factor of 5.
Reducing it to a factor of 2.5 (not like failure would result on something falling on me) it would be 1430 psi.
Pressure Rating of Steel Cylinder Tubing - Womack Machine Supply Company

Maybe your tubing is a better grade of steel or you have a much lower factor of safety?
 

Doug W

Hot Rolled
Joined
Sep 22, 2003
Location
Pacific NW
I think it would be easier to cut it apart, measure everything, and machine new duplicates and only salvage the honed tube.

Why would I throw away usable components?
On 1 cylinder the end cap can stay in place, only cut off the clevis mount.
And the piston, and gland nut can be reused.

On the 2nd cylinder the internal thread for the gland nut could be reused IF shrinkage from the welded on flange doesn't mess it up.
That is why I am asking.
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
Something like the quadram ripper off a D9H has 10" bore cylinders,and maybe 3/8" wall ......Ive scrapped ,salvged ,repaired ,and rebuilt cylinders ,and never done any kind of calculation of anything......in fact ,just "monkey see -monkey do"....The only thing ive taken a bit of interest in is the rods....sometimes they are 4140,most of the time rods are not hardenable,and quite soft ,relying on the hard chrome for wear and damage resistance.......For instance,you can oxycut drawbar pins from Caterpillar rods,and leave them laying in the dirt......they never rust...... John Deere ,the rods rust on new machines sitting in the open unsold.
 

Mike C.

Diamond
Joined
Nov 25, 2004
Location
Birmingham, AL
You are going to get distortion on the threads if you weld a flange around the tube. Just set it up with the steady rest (turn a pad around the tube if it doesn't have one already), weld on your flange and then chase the thread. Common work on tube threads when making a tube or repairing galled up threads.

As for the butt end, just make it the same as it was and add rod eyes. No big deal. Only issue I've ever seen that way was a huge boom truck (sign truck) cylinder that had a 3" od rod eye welded across the bottom. When they built the cyl, they tested it before the eye was welded across the butt. Lack of support caused the relatively thin butt (3/4") to bulge out. Had to cut the butt out and weld in a new flat piece. Made sure the eye was welded on before test that time.
 








 
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