Hydraulic cylinder seal friction.

[gyro]

I have constructed some small cylinders to actuate the rudder on an ultralight aircraft. They are 19mm and 12mm (3/4" and 1/2&quot bore diameters.
Two cylinders are at the pedals and two back at the rudder.
When in operation, all four pistons are moving at once as the opposite side cylinders are being pushed back by one pedal pressure.
The problem is; at very low rates of movement, pedal pressures are very high. They are quite acceptable once the pistons are moving at a reasonable speed.
The seals used are automotive brake mastercylinder seals, the type with a hole in the middle which locates them over a pin or post on the piston, and the fluid is brake fluid.
The distance that they travel in the cylinders is 3inches and 1.4inches. The cylinders are aluminium.
Would a normal cup seal, as in auto wheel cylinders have less friction and would they handle the use
Would it be possible to reduce the friction by using a different fluid and a totally different type of seal?
As this is a closed system the seals need to be pretty good or one may find there is no steering in flight!
I would be grateful for any advice

 

[ccfowler]

These are just thoughts and not based on any direct experience:

Did these seals work OK at first and then become troublesome, or have they always been troublesome?

My questions arise from considering that automotive brake parts have much shorter strokes and the cylinder material is usually steel or cast iron. Could surface hardness, finish, and stroke length be an unfortunate combination?

Could the cylinders be distorting? Could the pistons be binding by motion akin to buckling?

 

[EdDanzer]

What you are experiencing is slip stick, of break away friction. If a seal is doing its job the surface behind it is dry. Moving dry aluminum against dry aluminum is probably the problem. Try using a plastic bushing between the piston and bore.

 

[gyro]

I have been away for a while, thanks for the opinions.
The pistons were all stainless steel.
I have since tested a master cylinder off a motor vehicle that is also half inch dia bore and find that it also has the same drag when used in the same application. In normal brakes the seal does not encounter high pressure until it has the slave or wheel cylinder at the end of its travel, then the master cylinder moves very little further. In my case all the seals are moving with a constant high pressre.
It appears that I should be using a similar seal arrangement to that of a hydrualic ram but can you get that type of seal so small?
Gyro.

 

[EdDanzer]

One Problem you will encounter when looking for a different seal is fluid compatibility. If you change seal design (shape) it can effect the force required to move the piston under pressure as much as the material.
Rubber has a high coefficient of friction, Teflon would be a good choice for friction but has pressure limitations. Even though some manufactures quote high pressure, a seal will not perform well if the fluid pressure is greater than the yield strength of the material.
I think aluminum and stainless have a high coefficient of friction when used together.
Good luck with you redesign.

 

[Metalguy]

Motorcycle forks call this phenon. "stiction". Almost all forks are hard-chrome plated, and they still stick-even after hundreds of hours riding.

The lastest efforts involve coating the forks with something even harder-titanium nitride (TiN). They are golden brown color-don't know how well they work. There are companies that can coat your parts.

 

[gyro]

Thanks Eddanzer and Metalguy,
Reading your expert advice causes me to think that this is getting into the "Too complicated" category. However I now am much more knowledgable and may do a little more testing which may produce something.
Grateful regards, Gyro.

 

[atad]

There are some good quality, low friction seals on the market which use a bronze loaded PTFE Ring is in contact with the bore in a piston application and the piston rod in a rod sealing application. In each case the PTFE Ring is energised by an elastomer O-Ring. Try looking at Busak + Shamban or O. L. Sealing Systems. I think the brand name for this seal is Slydring in the case of Busak + Shamban.

 

[EdDanzer]

To look at different seal types try

http://www.herculeshydraulics.com

This company reps. several companies and sells in small quantities.
Changing seal type shouldn't be to difficult, and would give you a chance to change piston material.

 

[harrisj]

I agree with atad - go for a high tech (= expensive) seal from B&S or Greene Tweed.

When you've done this, there's no reason to use brake fluid which is a heritage from the golden days of motoring when seals had to be made from natural rubber - there wasn't anything else.

With nitrile, PTFE, fluorocarbons etc you can use any fluid you want - a good quality mineral oil based hydraulic fluid will have better lubricity than brake fluid (or use Citroen brake fluid which is mineral oil based).

Keep diameters small and pressures high - a good way to keep friction down.

John

 

[automatic2]

Disposable style pnuematic cylinders are rated at 250psi. Most use an aluminum piston with a stainless barrel. The piston will always have a plastic wear ring or bearing. Seals vary between Orings and cupped seals. These cylinders have low break away friction and excellent longevity. Use a good grade of hydraulic fluid. Check out the Clippard, Bimba, SMC lines, or a dozen others.

 

[budt]

Gyro;

You could cut your seal friction by removing the pistons and using the rods as single acting rams. This will work since the connection of the rams at the rudder synchronize them mechanically. You may need to use larger rams (rods) than you now have but that should be a minor problem.

Bud Trinkel
Certified Fluid Power Engineer
HYDRA-PNEU CONSULTING, INC.
812-853-3234
fluidpower1@hotmail.com