My last comment on the thing.
My comment above was, at the least, confusing. The seat that I spoke of, would of course have to have a central hole for the fluid to flow through, so it would be rather small in size (at least the size of the hole in the seats outer dimensions) then smaller in inside dimensions, taken into account the necessary hole. Would look more like a washer. Because of these very small requirements, displacing the seat would be easy. That is if there really is a separate seat-----I'm just guessing.
So, I intend to sit back, shut up, and watch the developments. Hang in there Dennis.

Chuck
05/XK8 Vert C.F. 1 of 200

 

Chuck,

I can't see very clearly in there, but it looks to me like no separate seat. IAC the appearance of things in there, and measurements, say it's the same as the two other valve bodies I have open ... one 110 Bar, and one 85.

The hole in the center where the ball sits is just shy of .01 square inches.

Hey, please don't stop talking to us if you have an idea. I am sure to need some ...

 

Just a thought and I have no real idea...could the spring have lost it's rate of compression when you bottomed it out? In other words it now does not have the same strength. Is it still the same length as when you first disasembled the valve....pulling at straws here...

 

For sure worth thinking about, But it still behaves like the other two springs I have to work with, as best I can measure them. (In theory, it should not be effected.)

Anyway, I think maybe it's time to stop with the noodling and do what's called a "proof of concept" test. If things in fact function like I'm picturing, it should be possible to remove the stock 110 Bar valve from the latch circuit, substitute a stock 85 Bar valve, and still have the system work. I'd like to do such a test, but the only spare 85 Bar valve I have has been subjected to me disassembling it.

Could anyone with some spare components loan me an 85 Bar valve? I promise not to poke at it in any way ... I'll just try it in place of the 110 and see what happens.

If things work, then it's all just fine tuning from there. If they don't work, then it's time to do a reset on the whole project.

Any help much appreciated.

 

Dennis - here's my two pence/cents worth based on what I remember of my Chemical Engineer training (way too long ago!) when it comes to how control valves operate.

First of all, based on the design of the valve, I think we are dealing with a pressure control valve not a pressure relief valve. The distinction is that pressure relief valves are usually designed to be either fully open or fully closed. In our case, the spring will cause the valve to modulate the flow in response to the pump outlet pressure.

Initially the valve will be closed until the pump pressure rises slightly above the spring force acting on the ball bearing. As the valve begins to open, the actual orifice is not the full diameter of the hole in the valve seat but the annulus formed between the diameter of the ball bearing and the edge of the valve seat. That complicates the calculation of the spring pressure required to have the valve lift at 110 bar (or whatever). The pump flow is essentially fixed so if the flow though the valve at a given spring position is too low, the system pressure will rise and in turn the spring will be compressed a little further thereby increasing the size of the annulus until the flow through the valve matches the output of the pump.

I understand your thinking about how drilling out the screw may have changed the flow through valve, but I personally think that may be a red herring. I'm much more inclined to think this is all about getting the spring tension set correctly - but I have been wrong before!

Anyway one suggestion might be to use a Volt meter across your battery as a proxy for pump power. If you monitor the voltage drop with the pump operating with your good valve, it might give you a base line for then testing the modified valve. If you were to gradually increase the spring tension until the hood works whilst keeping a close eye on the voltage drop it might give you the comfort you need that you could stop the pump before damaging anything.

Easy for me to say!

 

Bamforp,

Can't thank you enough!

I agree this must act like a control (vs. relief) valve. I finally saw yesterday that the spring force could not hold the ball on its seat all the way up to the marked pressure of 110 (or 85) Bar.

I have been stuck on the change that occurs when the ball first lifts off the seat. Before it lifts, the force from fluid pressure on it is the pressure times the area of the hole under it, agree? But an instant after it lifts, the whole "underside" of the ball sees fluid pressure ... much bigger area. Seems like the force on it would jump at that point. Is that close to what you're describing?

I'll check to see how large an annulus is formed by the area between the ball and the valve body. But as I picture what you're describing, it sounds like a tapered body would be needed ... how else can the size of the annulus formed around the ball increase as the ball rises through the valve body?

Point taken on the possible red herring. Thing is, I have checked the screw setting and positions of all the component in there so many times ...

Nobody said this would be easy!

Thanks again.

 

There's a nice illustration here of various valve trim characteristics.

My guess is our valve is operating like the middle illustration ie a linear characteristic. The valve is sized such that there is always a section of the ball that is 'inside' the seat over the range of operating conditions. That's how the annulus size changes as the ball moves in relation to the seat.

My take on the fluid dynamics are that when the valve is closed the force of the pressurised fluid is acting against the force of the compressed spring. When the pressurised fluid force exceeds the equilibrium point, the spring will compress slightly and valve will start to lift. At that instant, the potential energy of the pressurised fluid will convert to kinetic energy and cause a small flow of fluid towards the reservoir. When a portion of that flowing fluid hits the surface of the ball bearing, some of that kinetic energy gets converted back to potential energy and acts as a compressive force on the spring. In other words, once it starts to open, the forces acting against the spring are predominately related to flow rate not the pressure in the system. Now the flow rate is of course a function of the size of the annulus and the upstream pressure (assuming the reservoir pressure is constant) which are in turn related to the distance the ball moves away from the seat.

All of that to say that the maths gets complicated very quickly but ultimately the only practical variable we have in our case is the pre-load on the spring. Everything else is fixed by the design and sizing of the components. I cannot think of how any other variable would come into play.

Now the puzzle remains why when you set the pre-load back to the original setting does the valve not operate as expected? My guess is that very small movements of the screw translate into big shifts in the regulating pressure of the valve and that the valves are calibrated on a rig at the manufacturer rather than by relying on a fixed number of turns on the screw.

We don't have a such a rig, but I did wonder if using volts as a proxy for pump power could be used somehow to support a rough recalibration.

 

Thanks again. Learned a great deal there. As you said it all boils down to one or a few variables. I see these possible causes of the trouble:

- I haven't returned the valve components and adjusting screw to the correct positions. I have to (tentatively) put this one aside for now. Everything checked many times, and even if I missed the screw position by as much as, say, 1/2 flat (a huge miss) it should work since this would still be well above the 85-Bar screw position. (They may well be set at the factory by testing each one, but the two 110s I have were set to the same point, to within a small fraction of one flat. The 85 was set to about 2 flats less spring compression.)

- The drilling I did to free the adjusting screw does matter, it created a bigger effective orifice (seemed far fetched at first, but maybe ... )

- The old style valve is internally different from the new style or uses a different size orifice (also seemed far fetched, but there are several things different about the newer pump. Just maybe ... )

Without a stock 85-Bar, preferably new style, valve to drop in there and test I'm stuck. It may make sense to loop back to the idea of going to a commercial relief valve. But I'd still need to find an adapter for the 1/4" x 28 TPI threads used to mount the current valves. Weird thread; no components around.

Sound about right?


(Voltage measurements: this is the stuff I'm supposed to know about, and honestly I don't see any practical electrical measurement that is going to help us here.)

 

Have you considered taping off the hole you drilled with some electrical tape or similar? If it starts working then we have the answer. If not, and the tape remains intact then we know that flow through the drilled hole isn't the issue. If the tape has been blown off then we know that there is significant pressure behind the screw and that the hole needs to be sealed off more effectively.

 

Yessir! Another M.E. friend suggested a similar idea last night, along with some sealant on the adjuster threads.

When I can get back in there I'll try something like this, along with comparing the orifices in the old and new style valves. (But no way I'm taking that one apart!)

Sooner or later, one way or another, I'm gonna get this sumb*tch.

 

We're all pulling for you. I tried to buy a "junk" pump from a Jag recycler on ebay, but he said all of his pumps were "good". I just want those valves!

 

So we learn more from failure than from success, right? If so I'm learning a whole lot lately!

The photo shows an "old style" 85-Bar valve in which the hole originally drilled to free the adjusting screw has been tapped and a 4-40 screw inserted. This, thought I, killed two birds: locks the adjusting screw, seals against possible fluid leaking through the hole, which might make a bigger effective orifice. PTFE sealant on adjuster and locking screws.

This valve was put in a new-style pump, in place of the 110-Bar latch-circuit valve. Same trouble as before; not enough pressure being developed for the top to work.

I have no spare new-style valves to experiment with, and think I should now abandon trying to get an old-style valve to work in there. Will return to trying to fit a commercial relief valve replacement. I think we could keep the cost of this option under $50.


But I need to put this down for a couple of weeks or so, and clear my head. Sorry I don't have a working prototype to report at this point.

 

The LSI valve is still available.

 

Dennis, Thank you for your dedication to this project. Some times a break can give us a better direction when we return.
Enjoy your car for the next couple of days and we await your return to pressure. LOL


Wayne

 

Personally I'd favour the LSI valve pressure reduction solution over the resistor method, but if Dennis can find a way to make the internal valve perform the same function that would be my preferred option.

 

OK, so I've got my car working with reduced peak pressure in the convertible top's hydraulic latch circuit (the one prone to failure). This was achieved by modifying the pump's internal relief/regulator valve. I haven't modified the ram circuit at this point, but the same technique can be used there.

The photo below shows both the stock spring contained in the valve, and the replacement used for this mod. While they look similar, the stock spring is nearly twice as stiff as the replacement (spring constant ~ 425 lbs per inch v. ~ 225 per inch). The different spring coupled with a change to the adjuster screw setting on the valve create a max pressure of about 1150 PSI in my test system. (That's a calculated value; I don't have a way to measure pressure directly, but the only fuzzy values in the calculation are the spring constants, and I think I've been able to measure them to within 10-15%.)

The 1150 PSI can probably be reduced further (a change to adjuster screw setting), but I wanted to be conservative to prove-in the concept.

The mod works with or without the voltage-reducing resistor, but since that component reduces stress on the hoses in a different way, I will leave the resistor in place on my car.


The required replacement spring costs about $2.50. I'll do a more complete write-up with a source for springs and instructions for doing the mod later this week.


Sincere thanks to everyone who helped and offered encouragement. A special tip-of-the-hat to Bamforp for the (much needed) education on the workings of valves.

Sure am glad this worked out. I was beginning to wonder ...

 

Well done you Dennis

 

You Da Man!!
Many questions, but will wait for your coming write up.

Chuck
05/XK8

 

Still wish we had a bench tested valve to install rather than counting turns.

I suspect that is the only way this is going to going to be significantly successful.

 

Tom,

No question an actual pressure reading in situ would be best. Maybe someone will oblige us.

It worked out that counting flats (to pre-load the spring) is required, but the overall pressure reduction is not very sensitive to this. The softer spring makes most of the difference. I'll try to post some sort of graph showing this stuff next time.