Gang,
Some of you out there are pretty good with hydraulics, so here
is something I need a little help with....
I have a chunk of aluminum, with a bore of .696 inches. I need to
make a small piston with two "o" rings around it to fit down into the
bore to be used with hydraulics. Since the main device is aluminum,
I wanted to make the piston from steel, but I'm not sure of the kind
of clearance I should have. I was thinking of about a thou, then I
was thinking that when the steel and aluminum got warmed up, that my
clearance would diminish enough that the piston would bind up in the
bore. Any advice? Suggestions? Thanks in advance.
BTW, hydraulic pressure will only be applied to ONE end of the piston
and it will be returned to original resting position by a spring
behind the piston.
Ken.

Ken Sterling (Ken Sterling) wrote in message
...
Gang,
Some of you out there are pretty good with hydraulics, so here
is something I need a little help with....
I have a chunk of aluminum, with a bore of .696 inches. I need to
make a small piston with two "o" rings around it to fit down into the
bore to be used with hydraulics. Since the main device is aluminum,
I wanted to make the piston from steel, but I'm not sure of the kind
of clearance I should have. I was thinking of about a thou, then I
was thinking that when the steel and aluminum got warmed up, that my
clearance would diminish enough that the piston would bind up in the
bore. Any advice? Suggestions? Thanks in advance.
BTW, hydraulic pressure will only be applied to ONE end of the piston
and it will be returned to original resting position by a spring
behind the piston.
Ken.

How much pressure? How long stroke? Why steel? Why .696"? Blow out the
bore to .75" would allow you to use off-shelf parts.

In article , Ken Sterling (Ken
Sterling) says...
Gang,
Some of you out there are pretty good with hydraulics, so here
is something I need a little help with....
I have a chunk of aluminum, with a bore of .696 inches. I need to
make a small piston with two "o" rings around it to fit down into the
bore to be used with hydraulics. Since the main device is aluminum,
I wanted to make the piston from steel, but I'm not sure of the kind
of clearance I should have. I was thinking of about a thou, then I
was thinking that when the steel and aluminum got warmed up, that my
clearance would diminish enough that the piston would bind up in the
bore. Any advice? Suggestions? Thanks in advance.
BTW, hydraulic pressure will only be applied to ONE end of the piston
and it will be returned to original resting position by a spring
behind the piston.
Ken.



With a steel piston in an aluminum housing the clearance will increase
as the temperature rises. Your .001 clearance is about right for a close
sliding fit at that size - MH lists shows .0006 to .0023 clearance for
an RC4 (close running fit) from .400 to .710 diameter.

If there are no large eccentric loads on the piston, you should be able
to make the piston smaller and rely on the seals to keep it centered and
eliminate metal to metal contact. This is pretty common in small
pneumatic valves with aluminum spools and die cast bodies.

Ned Simmons

using plain o-rings for a hydraulic piston will require a very close fit
to keep the extrusion gap acceptable for o-rings

my recommendation would be a standard .75 bore , a piston with a wear
band to keep it centered in the bore regardless of load and a polypak
type u-cup seal on the oil side of the piston

I just checked, there's a wonderful discussion of O-ring design in
Machinery's Handbook. About three pages with tables.

Karl

Be aware orings are fatter than stated. For example a 1/8" oring is
..139".

With that said, the groves in the piston should be deep enough to
compress the oring around 25% against the bore. The groves should be
wide enough for the squashed o-ring to have somewhere to go.

So, an 1/8" oring (really .139) should have clearance between the OD of
the groove and the ID of the bore of .104". The groove would be around
3/16" wide.

The oring should be stretched some in its grove, but only minimally,
like 5%.

A rough bore will ruin the oring. Not a bad idea to hone it in the
direction of use. A bright finish is not necessary, just no sharp
roughness.

A lead in, chamfer or radius to the bore will prevent pinch cutting the
ring during install.

Polymer Man wrote:
Be aware orings are fatter than stated. For example a 1/8" oring is
.139".

With that said, the groves in the piston should be deep enough to
compress the oring around 25% against the bore. The groves should be
wide enough for the squashed o-ring to have somewhere to go.

So, an 1/8" oring (really .139) should have clearance between the OD of
the groove and the ID of the bore of .104". The groove would be around
3/16" wide.

The oring should be stretched some in its grove, but only minimally,
like 5%.

A rough bore will ruin the oring. Not a bad idea to hone it in the
direction of use. A bright finish is not necessary, just no sharp
roughness.

A lead in, chamfer or radius to the bore will prevent pinch cutting the
ring during install.


an o-ring makes a horrible dynamic seal , get a u-cup type seal ,

but for an 1/8 cross section o-ring [.139 ]

the width of the groove should be .179 - .186

the depth on the radius from the bore dimension should be .110 - .113

the clearance between the bore and the piston o.d should not exceed .003
total or .0015 per side

if you want to use a back up ring or an anti extrusion device the groove
would be widened accordingly

the surface finish on the bore and the groove should be 10-20 micro
inches for best seal life but can get as high as 32

the finished surface marks should all be perpendicular to the direction
of travel

not hatched like an engine that uses cast rings


I recommend everyone that plans on doing hydraulic work get a set of
catalogs from Hercules bulldog , full of good information


18003335617 option 4

or order on line

How much pressure? How long stroke? Why steel? Why .696"? Blow out the
bore to .75" would allow you to use off-shelf parts.

I may not be able to give definitive answers to your questions, Tom...
but here's a quick and dirty on what I'm up to.
Picture a track mounted backhoe (small size called a mini-excavator).
This unit has a dozer blade for pushing the dirt back into the trench
upon completion of the job. The dozer blade leaks off after raising it
up with the control. I checked the cylinder and with the blade raised,
and both the cylinder ports plugged, the blade stays in the raised
position. I thought it was the valve, but upon further inspection, no
problem in the valve either. This unit is made in Japan, sold in
England, and there is not much info in this country (USA) concerning
it, however I had been e-mailing some guy in England and was informed
that my problem was more than likely the "rotary coupling" used in the
center of the machine to couple the upper framework (which can rotate
360 continuously) to the lower framework (which consists of the
tracks/motors and the dozer blade/cylinder). This prevents hydraulic
hoses from winding around in the machine. I understand the rotary
coupling will permit fluid to circulate and the blade to drop.
Replacing this coupling would be a monumental task, totally out of my
realm of possibility (and extremely expensive), so I decided to come
up with a different idea.
The only kind of a check valve I can seem to locate is a piloted
check, which I don't want to use. So I am making a block of aluminum,
with 1/4" pipe connections for input and output. From EACH of the 1/4
pipe fittings, there will be a channel going to the center of the
block and appearing at the bottom of a flat bottomed hole. A piston,
with a stout spring will push down on the flat bottom of the hole and
"seal off" the two channels going to the 1/4" ports. The amount of
pressure needed is only a few pounds to prevent the oil from
circulating around and letting the blade drop. As the operator lever
is moved, hydraulic pressure from the pump will overcome the little
piston (doesn't matter which DIRECTION of flow) and the blade will
raise or lower as needed. When the lever is released, my little
piston will seal off the two holes again, holding the blade in
position. That's it. I came up with the size as my "channels" are
7/32" holes, spaced 1/8" apart from each other in the center of the
piston bore, with a little bit around the outside. It can be any size
I need, but the smaller the bore, the less pressure I will need to
seal the ports.
Am I crazy? or will this work.....
Ken.

In article , Ken Sterling (Ken
Sterling) says...
Gang,
Some of you out there are pretty good with hydraulics, so here
is something I need a little help with....
I have a chunk of aluminum, with a bore of .696 inches. I need to
make a small piston with two "o" rings around it to fit down into the
bore to be used with hydraulics. Since the main device is aluminum,
I wanted to make the piston from steel, but I'm not sure of the kind
of clearance I should have. I was thinking of about a thou, then I
was thinking that when the steel and aluminum got warmed up, that my
clearance would diminish enough that the piston would bind up in the
bore. Any advice? Suggestions? Thanks in advance.
BTW, hydraulic pressure will only be applied to ONE end of the piston
and it will be returned to original resting position by a spring
behind the piston.
Ken.



With a steel piston in an aluminum housing the clearance will increase
as the temperature rises. Your .001 clearance is about right for a close
sliding fit at that size - MH lists shows .0006 to .0023 clearance for
an RC4 (close running fit) from .400 to .710 diameter.

If there are no large eccentric loads on the piston, you should be able
to make the piston smaller and rely on the seals to keep it centered and
eliminate metal to metal contact. This is pretty common in small
pneumatic valves with aluminum spools and die cast bodies.

Ned Simmons
Thanks, Ned. This information helps.
Ken.

using plain o-rings for a hydraulic piston will require a very close fit
to keep the extrusion gap acceptable for o-rings

my recommendation would be a standard .75 bore , a piston with a wear
band to keep it centered in the bore regardless of load and a polypak
type u-cup seal on the oil side of the piston
In this particular application, a u-cup (although providing perfect
sealing qualities) won't work. I need a flat surface to seal off two
holes in the bottom of a flat bottomed hole.
Thanks.
Ken.

Be aware orings are fatter than stated. For example a 1/8" oring is
.139".

With that said, the groves in the piston should be deep enough to
compress the oring around 25% against the bore. The groves should be
wide enough for the squashed o-ring to have somewhere to go.

So, an 1/8" oring (really .139) should have clearance between the OD of
the groove and the ID of the bore of .104". The groove would be around
3/16" wide.

The oring should be stretched some in its grove, but only minimally,
like 5%.

A rough bore will ruin the oring. Not a bad idea to hone it in the
direction of use. A bright finish is not necessary, just no sharp
roughness.

A lead in, chamfer or radius to the bore will prevent pinch cutting the
ring during install.

Good info - I can use it. Thanks.
Ken.

Ever consider a piston for a model airplane engine. Some use rings but
you may have to change the bore dia Some are steel, others aluminum or
cast iron.

Good suggestion. The piston I'm planning on using would be fairly
simple, though. I think steel piston in an aluminum bore with o-ring
seals in two places around the piston. Thanks.
Ken.

I just checked, there's a wonderful discussion of O-ring design in
Machinery's Handbook. About three pages with tables.

Karl


Thanks for the info Karl, I have the 19th edition and will look
through it.
Ken.

I just checked, there's a wonderful discussion of O-ring design in
Machinery's Handbook. About three pages with tables.

Karl


Thanks for the info Karl, I have the 19th edition and will look
through it.
Ken.
Karl,
It appears the 19th edition has nothing on o-rings.... :-(
Ken.

On Wed, 19 Apr 2006 10:49:06 GMT, Ken Sterling (Ken Sterling) wrote:

I just checked, there's a wonderful discussion of O-ring design in
Machinery's Handbook. About three pages with tables.

Karl


Thanks for the info Karl, I have the 19th edition and will look
through it.
Ken.
Karl,
It appears the 19th edition has nothing on o-rings.... :-(
Ken.

Just Google Parker's on-line O-ring manual. I have it in hard copy. A
bearing house ought to be able to get you a copy.

If you want to download the manual, go to
http://www.parker.com/Catalogs/ and open the catalog, then save it.
Click on each new topic. When a new file opens, save it. There are
12 files labelled 00-5700.pdf through 11-5700.pdf. When you have them
all saved to a folder, you can open the first one and navigate to the
rest by the links in the left frame.

Pete Keillor

On Wed, 19 Apr 2006 10:49:06 GMT, Ken Sterling (Ken Sterling) wrote:

I just checked, there's a wonderful discussion of O-ring design in
Machinery's Handbook. About three pages with tables.

Karl


Thanks for the info Karl, I have the 19th edition and will look
through it.
Ken.
Karl,
It appears the 19th edition has nothing on o-rings.... :-(
Ken.

Just Google Parker's on-line O-ring manual. I have it in hard copy. A
bearing house ought to be able to get you a copy.

If you want to download the manual, go to
http://www.parker.com/Catalogs/ and open the catalog, then save it.
Click on each new topic. When a new file opens, save it. There are
12 files labelled 00-5700.pdf through 11-5700.pdf. When you have them
all saved to a folder, you can open the first one and navigate to the
rest by the links in the left frame.

Pete Keillor
THANKS!, Pete.
Ken.