AHC accumulator replacement from McMaster? (1 Viewer)

[SerasLibre]

Has anyone ever considered replacing their failed AHC accumulators with something similar to these units sold in McMaster?

Scroll to the bottom of the page in the link to reveal the Diaphragm-style accumulators. I'm not 100% sure of the size of our globes, but I assume they are about 1 quart. Buying a set of 4 + the charging and gauging assembly should allow you to recharge your globes after they depressurize, at a total cost of $1250. Cheaper than the B&B option, but untested at this point.

Note that there are a few variables to consider which I am not completely familiar with; The McMaster accumulators handle 3,000psi max and have an SAE-type fitting. There is also a flow-issue which means that the port size of the accumulator must be equal to or larger than that of the OEM one.

Has anyone given this some thought? I own an 05' LX whose globes WILL eventually fail. If something like this can help us refill them after a few years, even if you can only refill them twice (or 3x) before they fail, they are worth the investment. Afterwards you would only need to buy new accumulators (4 for $891), not the charging kit.

Any thoughts?

 

[Tinkerer]

I wouldn't try using Accumulators not specifically designed for the LX suspension.

The effective spring rate presented to the LX hydraulic "shock" is a result of Accumulator geometries including diaphram area/shape and gas volume, plus gas charge pressure. You could vary pressure but not the geometries. Spring rate (dynamic) and damping are also affected by fitting/line/etc sizing. And finally, long-term, the non-automotive type Accumulator might not be able to handle automotive conditions such as vibration, severe thermal cycling, corrosion environments exposure, etc., plus they might not be able to handle the higher duty cycle (compression cycles per unit time and amplitudes) presented by the LX suspension in operation, compared with a typical industrial hydraulic application.

Bottom line: With non-OEM type Accumulators, you would spend lots of time playing around with charging pressures to get the system to perform anywhere near what it should, it probably would still not ride like it should, and these Accumulators would likely not last as long as OEM in the LX application - and a failure might be a rupture creating a hydraulic (AHC) fluid mess.

 

[uHu]

Tink: No positive thoughts today?

Seras: Great idea.
Here below are some details of the gas springs (aka spheres).
One problem though is the space. If alternative spheres are any bigger, or differ in shape, it might not be possible to fit them without a piece of pipe to the actuator. Or maybe relocate the actuator, but there are pipes going in to them too.

Specs:
The front gas chambers have a gas volume of 400 cc, and a pressure of 2.26 MPa (327 psi).
Rear chambers volume 500 cc and 2.65 MPa (384 psi).

And while we are at it:
The Accumulator (cylindrical thingy on the left rail) is a free-piston type with dual piston rings, filled with nitrogen for pressure. The nitrogen volume is 945 cc with a pressure of 5.9 MPa (853 psi).

The Pump Attenuator is also a kind of accumulator, with a bellows filled with nitrogen, 2 cc at 1.96 MPa (284 psi).

 

[sonk76]

Tink: No positive thoughts today?

Seras: Great idea.
Here below are some details of the gas springs (aka spheres).
One problem though is the space. If alternative spheres are any bigger, or differ in shape, it might not be possible to fit them without a piece of pipe to the actuator. Or maybe relocate the actuator, but there are pipes going in to them too.

Specs:
The front gas chambers have a gas volume of 400 cc, and a pressure of 2.26 MPa (327 psi).
Rear chambers volume 500 cc and 2.65 MPa (384 psi).

And while we are at it:
The Accumulator (cylindrical thingy on the left rail) is a free-piston type with dual piston rings, filled with nitrogen for pressure. The nitrogen volume is 945 cc with a pressure of 5.9 MPa (853 psi).

The Pump Attenuator is also a kind of accumulator, with a bellows filled with nitrogen, 2 cc at 1.96 MPa (284 psi).

Wow! Someone knows a thing or two about the AHC system. Very nice.

By any chance do you know of any diagrams that might help visualize the internals of any of the AHC system components (e.g., the globes, etc.)? While I really appreciate the verbal descriptions, I have a hard time visualizing/understanding how it all works. Would love to understand it all better.

 

[uHu]

Somebody in here made a simplified functional drawing. Can't remember who or where.

 

[hoser]

Image 1 shows the action (click on for animated GIF). Blue being the nitrogen gas and green, the hydraulic fluid. Image 2 is more like our system as the spheres are located separately from the shock.

 

[agaisin]

There are other manufacturers that use similar globes and the part that stinks is that the globes for the other cars are cheap - like 100 bucks or so. Mercedes, BMW, Citroen. Aside from specs of the diaphragm, psi, volume, the biggest physical problem is matching the fittings to our car (assuming the other globes fit).
Here's a writeup for bmw that could easily be mistaken for the lx: Suspension - LAD accumulator replacement

It would be awesome if someone found a way to use other globes with our trucks.

 

[agaisin]

Another source for diaphragm style accumulators: Data sheet for AccuMight® Diaphragm Accumulator -- AM15031003 from Accumulators, Inc. in Pulsation Dampeners on GlobalSpec

 

[Tinkerer]

Tink: No positive thoughts today?

Positive thoughts where merited, pragmatism always. I've converted back to the philosophy of K.I.S.S. these days, by necessity. (Darn AHC is too expensive to keep in good operating condition for the long term, even with B&B or non-automotive Accumulators. "Dealers" aren't called "Stealers" for nothing.)

If someone replaces OEM LX Accumulators with non-automotive industrial hydraulic Accumulators, please post how it goes. (And please post your general whereabouts, year/color LX, etc. - I will want to make sure I am never near your vehicle in operation on the road. An AHC hydraulic system rupture at a couple thousand psi going down the highway could be spectacularly messy, and possibly result in an accident & a "Car-B-Que" started with hydraulic fluid.)

 

[SerasLibre]

Tink - your concern is well founded. I agree that toying with this idea may be risky if not done right, but the reward makes it worth the effort in my opinion.

uhu - Do you know what the port size is and what type of fitting is used for the globes?

agaisin - The issue with AHC, and limited other height-control systems (think Range Rover air bag system) is that they must allow for high suspension travel - unlike on sedans.

Another question - is the suspension fluid a silicone-based oil, petroleum, or synthetic? Has anyone performed a fluid analysis on it? This will determine any bladder compatibility issues that will have to be considered.

One more thing - the only reason a bladder is needed at all is because of the position of the accumulator. If it were upright, we could potentially use a bladder-less accumulator. Since air is lighter than the fluid, as long as the port to whatever chamber is used is at the bottom of the container, air will never leave the port. The main concern with a setup like this, however, is that implementing such a setup would require high pressure plumbing from the actuator valve. This plumbing will introduce a resistance to flow that will affect the rate at which fluid moves through the valve. However, it is likely that by stepping up the size of the tubing at the outlet to the accumulator, this effect can be negated.

Any hydraulic experts out there willing to chime in?

 

[scottm]

A diaphram accumulator is a diaphragm accumulator. Unlikely you'll find one that's not compatible with automotive fluids. There are a lot of cars and trucks running them, many cars have hydraulic brake assist, their "brake bombs" are a common problem. When I researched replacing the brake bomb on my Audi I found many cars shared the same accumulator, from domestic to Jaguar to BMW, and prices for the same part varied dramatically among the brands of cars. You might find this same accumulator on another vehicle for cheaper. And I was looking at McMaster as well, if it'll fit it's a great alternative. It wouldn't fit on my Audi.

 

[Yotato]

If you really want an aftermarket one, then check some of the BMW forums for aftermarket accumulator solutions. There are a few out there. I'm sure one of the listed mfg's can make one to specs.

 

[agaisin]

agaisin - The issue with AHC, and limited other height-control systems (think Range Rover air bag system) is that they must allow for high suspension travel - unlike on sedans.
QUOTE]

If you're suggesting that AHC adds complexity to the "globes" I'd argue that's not true.

First, on terminology (according to my understanding so I welcome corrections):
- AHC = Active Height Control
- AVS = Adaptive Variable Suspension
- Accumulator = there are 5 accumulators on the LX470/LC100. 4 are specifically to control damping force related to AVS (comfort/normal/sport1/sport2), the 5th I believe is a reservoir used when quick adjustments to either height or damping force are needed (accuracy?).

I don't think the 4 accumulators we're talking about here have any AHC specific technology to them - they are simply spheres with nitrogen on one side and a membrane on the other and they are threaded into the actuators where all the electronics and complexity are. Sure there are important specifications I don't know enough about like the volume of nitrogen and volume of luid and max psi it can handle and type of fluid that is compatible with the membrane, but there are no electronics in the globe itself - it screws into the complex parts.

Based on my understanding I don't see what AHC has to do with the 4 damping force accumulators. Now, the rest of the system (pumps, electronics, actuators, etc) - sure that all adds to the expense of the system, but as far as the 4 spheres everyone always *itches about I don't think they are inherently different than other globes.

The above is based loosely on observations and based on what I've seen in the FSM for Electronic Suspension Diagnostic for Suspension and Axle so I could be wrong. Of course I can't ever find any truly detailed documents about the way it all works and why so I think most of us are speculating here, just with varying degress of "educated guesses". I come from a software background and the lack of design documentation around components of complex systems that we pay for bugs me. I'd love to see detailed designs and white papers etc on how AHC and AVS work and why they work the way they do. If anyone knows of such documentation please forward links!

 

[scottm]

A hydraulic accumulator stores fluid and energy, and damps pressure surges. It supplies pressurized fluid to the system when the pump can't keep up with demand, allowing a smaller pump. It's not complicated or tricky to choose an accumulator when I'm designing an industrial machine, pretty simple stuff. Get something approximately the same size or bigger. You can mount it remotely and run a hydraulic line down to the system pretty simply, many cars have their brake accumulators mounted that way.

 

[hoser]

Agaisin, your terminology is correct. I think the confusion comes from the fact most members refer to "AHC" as the AHC/AVS/skyhook TEMS suspension as a whole and not the sole function of height control.

 

[SerasLibre]

Yes - although the globes are similar for all hydraulic-based suspension systems, the suspension on the hundy has a large amount of travel compared to sedans, so we likely use larger globes to allow for the fluid to (almost) fill the globe chamber when the suspension is fully compressed and hitting the bump stops.

Unless somebody knows the port size/type, does someone have a bad accumulator they could ship me?

Also - someone please correct me if I'm wrong, but my assumption as that for our purposes, getting a smaller globe than is factory installed may be a problem (potentially not enough room for fluid when suspension is fully compressed - likely to damage hydraulic "shocks"), but using a larger globe should not be an issue (suspension stops limit travel, not the globes). Afterwards all we need to do (besides finding a good spot for them if they don't fit) is testing with a range of air/nitrogen pressures to try to get the same response we do from the OEM system.

Does anyone know enough about Mercedes to tell if they used a hydraulic suspension on the S500 in the mid-90s? I'm pretty sure they did. If its large enough these may work.

 

[parnoren]

The Citroen Globes are very affordable. I don't know who makes them, but I think they are threaded.
SPHERE SUSPENSION FRONT HYDRACTIVE CITROEN XANTIA - eBay (item 180277317989 end time Jan-01-11 05:32:21 PST)

 

[agaisin]

Yes - although the globes are similar for all hydraulic-based suspension systems, the suspension on the hundy has a large amount of travel compared to sedans, so we likely use larger globes to allow for the fluid to (almost) fill the globe chamber when the suspension is fully compressed and hitting the bump stops.

Unless somebody knows the port size/type, does someone have a bad accumulator they could ship me?

Also - someone please correct me if I'm wrong, but my assumption as that for our purposes, getting a smaller globe than is factory installed may be a problem (potentially not enough room for fluid when suspension is fully compressed - likely to damage hydraulic "shocks"), but using a larger globe should not be an issue (suspension stops limit travel, not the globes). Afterwards all we need to do (besides finding a good spot for them if they don't fit) is testing with a range of air/nitrogen pressures to try to get the same response we do from the OEM system.

Does anyone know enough about Mercedes to tell if they used a hydraulic suspension on the S500 in the mid-90s? I'm pretty sure they did. If its large enough these may work.

I think I hear what you're saying just not sure if this is true. Whether I'm at height of 8" or at height of 12", wouldn't the travel over the same bump be the same (roughly)?

Also, note that while the AHC allows the height to go up or down, above 19mph (iirc) it is forced into N mode so the vehicle suspension is not designed to be in H or L at > 19mph.

 

[uHu]

Here are some measurements of the globes. Looks like the threads are M22 x 1.5, while the thicker part of the shaft is 24 mm. Length 26.5 mm. Text imprinted says "MUV200-500".
Pictures are the rear ones.

 

[uHu]

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