Venting a pull-type slave cylinder?

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greyhounds_amx
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Venting a pull-type slave cylinder?

Post by greyhounds_amx » Thu Jun 14, 2018 2:10 pm

Has anyone tried venting the back side of a Wilwood or similar pull-type slave cylinder?

There's a captive air pocket in there that you have to compress when you push the pedal. Wilwood makes an effort to minimize the extra force required by machining a recess into the piston of the slave, making the initial volume a little larger so the change in volume is reduced. As far as I can tell none of the other pull-type slaves out there even go through that level of effort.

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I'd like to lighten the pedal a bit if possible and based on a quick calc it looks like even on the Wilwood compressing that captured air adds up to 15-20 lbs of additional leg force that wouldn't be needed if is was a traditional push slave.

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Re: Venting a pull-type slave cylinder?

Post by Kevin Johnson » Sat Jun 23, 2018 4:42 pm

Degas your hydraulic fluid prior to filling the cylinder or reservoir. At STP the fluid will likely accept about 9% by volume. Here is a search if you want to read more about gas solubility in fluids:

https://www.google.com/search?q=gas+sol ... e&ie=UTF-8

Henry's law:

https://www.google.com/search?rlz=1C1CH ... ZcmDpqii30

Degassing techniques:

https://www.google.com/search?rlz=1C1CH ... jyE2ItzSKE
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ChevyEFI
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Re: Venting a pull-type slave cylinder?

Post by ChevyEFI » Sun Jun 24, 2018 1:26 am

Ideally, an in-service cylinder should have a way for the air to easily rise to the top of the system.

Or be bled from a high point in the system. I think you're trying to add a bleeder for this purpose? As long as the piston and external sealing is not potentially running on an unsmooth surface because of this change, it should be okay to add an opening for the purpose. A hose with a one-way brake bleeder is a nice luxury on some applications, with a tiny bit of added volume.

However, you hypothesize (if I am understanding you,) a change in trapped air will improve and lighten pedal feel. Simply comparing before and after of a vehicle that needed bleeding, I think you'll be disappointed,

Your answer is probably going to a smaller master bore, or smaller slave bore. If travel is adequate for release, that is going to be the effective answer.

What is the application?

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Re: Venting a pull-type slave cylinder?

Post by peejay » Sun Jun 24, 2018 12:12 pm

I think people are not understanding the problem you are trying to solve.

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I don't see why you shouldn't add a vent other than possible corrosion concerns, because if moisture got in there, it would have no way to get back out, and aluminum turns to high-volume powder very quickly. And obviously you can't put grease in there or else it will take up even more volume.

If you wanted to be really picky and keep it "sealed", you could drill and tap for 1/8" pipe thread and connect that to another sealed chamber. It will still be pushing the piston against a closed volume but at least the volume of air that you are compressing will be much larger.

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Re: Venting a pull-type slave cylinder?

Post by Kevin Johnson » Sun Jun 24, 2018 2:33 pm

When a gas goes into solution in a fluid the resulting mixture acts as an incompressible fluid. There is a certain amount of gas that will dissolve at a given ambient pressure.

Some engineers forgot about saturation points when attempting to use (say) in situ engine oil as a hydraulic fluid to actuate mechanisms whilst expecting simultaneity. You can check the patent records to see the tacit admission of same.

The key point is to enquire what the gas solubility is for the particular hydraulic fluid being used. The clutch slave cylinder is not subjected to the temperature extremes that say a braking system is (creating partial pressures from heated components of the fluid that would compete with the dissolved gases). The volume of trapped gas appears to be minor.

The mechanical advantage of the levers/chambers involved is a separate issue, yes. So would the composition and construction of the flexible/rigid hydraulic line. It certainly is an advantage to have bleed valves installed at the topmost point of systems and videos online illustrate their use with Wilwood slave cylinders.

A little basic physical chemistry goes a long way in providing simplified solutions in engineering which is why it is important that type of course be included in the degree curriculum.

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Re: Venting a pull-type slave cylinder?

Post by MadBill » Wed Jun 27, 2018 9:50 pm

greyhounds_amx wrote:
Thu Jun 14, 2018 2:10 pm
Has anyone tried venting the back side of a Wilwood or similar pull-type slave cylinder?

There's a captive air pocket in there that you have to compress when you push the pedal. Wilwood makes an effort to minimize the extra force required by machining a recess into the piston of the slave, making the initial volume a little larger so the change in volume is reduced. As far as I can tell none of the other pull-type slaves out there even go through that level of effort.

I'd like to lighten the pedal a bit if possible and based on a quick calc it looks like even on the Wilwood compressing that captured air adds up to 15-20 lbs of additional leg force that wouldn't be needed if is was a traditional push slave.
An air pocket will not affect the required pedal force, it will merely make the system a little 'spongy' as it compresses before liquid displacement occurs, i.e. instead of say 6" of pedal travel to get full disengagement it might take 6-1/2 or 7".

The only ways to reduce the pedal force for a given clutch are a larger slave cylinder, a smaller master or an increase in the mechanical leverage ratio.
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Venting a pull-type slave cylinder?

Post by Kevin Johnson » Thu Jun 28, 2018 8:08 am

PV = nRT Ideal Gas Law

If a coiled spring were added (like an additional or higher rate clutch diaphragm spring) additional force is required to overcome the sponginess.

~~~~~~~~~~~~~~~~

Under compression the ambient pressure rises making dissolution more felicitous. Dissolution may take some time as the bubble boundary dimensions (surface area to volume ratio) in contact with the liquid phase are important. The key idea is to degas the fluid prior to use. It is a lot of trouble, yes.

In the past I have mentioned sparging liquid fuel to replace dissolved gases with no fuel value (say nitrogen) with something that has potential fuel value. This is a "speed secret" but don't let regulators catch you doing it to spec fuel.


http://www.hydraulicspneumatics.com/hyd ... nerate-air

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Edit to the above Sidebar: the presence of dissolved gases does affect the volume (the additional "not" is a typo). About 40 years ago my high school chemistry teacher suggested an empirical experiment to determine whether dissolved materials take up additional volume. Place a glass of water filled to the brim on your Mother's favorite antique wood table. Slowly add sugar crystals, letting them dissolve and see if it spills over.

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Re: Venting a pull-type slave cylinder?

Post by peejay » Sat Jun 30, 2018 3:31 pm

Guys.

The air pocket is not in the fluid.

The air pocket is on the backside of the piston, it is a pull type slave cylinder so the piston is pulling the rod INTO the housing, therefore there is a sealed off space on the opposite side of the motion assembly. This space is full of air that must be compressed when the piston moves.

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Re: Venting a pull-type slave cylinder?

Post by Kevin Johnson » Sat Jun 30, 2018 10:42 pm

peejay wrote:
Sat Jun 30, 2018 3:31 pm
Guys.

The air pocket is not in the fluid.

The air pocket is on the backside of the piston, it is a pull type slave cylinder so the piston is pulling the rod INTO the housing, therefore there is a sealed off space on the opposite side of the motion assembly. This space is full of air that must be compressed when the piston moves.
Yes, rereading the original post with this interpretation does lead to a different answer. Sorry about that. I do not suggest adding a small vent as you would be performing additional work pushing air through an orifice and the system would repeat this by drawing air back through when it was released. Any potential variance in the rate of or degree to which the clutch re-engages* is not a good idea in a race car. With the current design an efficient air spring is created.

*Corrosion narrowing the opening or, worse yet, detritus of any sort clogging it on the return engagement stroke; fine grit from the track causing bore wear; rain/moisture gradually filling the chamber.

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