This post is my notes to myself, because whenever I get an air table in with the valves plumbed wrong I have to figure it out each time. I decided to share with the world because this is a needless source of frustration…and so I could access this page from somewhere besides my normal workshop.
Symptom: Your air-isolated vibration reduction table has a section that will only inflate, won’t deflate, and the height can’t be adjusted
Cause: The Height Control Valve is installed incorrectly
These valves have one fitting that is supposed to be the air supply inlet, and three fittings for height-regulating air pressure outlets (or isolator pressure gauges, if they are installed). I just had a Micro-g vibration isolation table come through with the pressure inlet plumbed to the isolator piston instead of the air inlet. As you will see, this was the cause of my frustration. First, let’s get the terminology straight:
Without the lever arm or table blocking your view, they look like this:
The insides of these are pretty straight-forward. They are a machined brass block with a valve to open or close the air supply to the isolator pistons. With the top off, you can see the plumbing:
The air supply is fed to the inlet of the valve, then the outlet of the valve (the hole in the center of the part) feeds a cavity with holes drilled, which connect to threaded fittings to be plumbed into isolators. The stem poking up out of the middle is the valve actuator. Press down on that, and the air (or nitrogen) pressure from the supply fitting is released to the (holes for the) isolator pistons. Most tables will have 3 “master” isolators and there will usually be at least one “slave” isolator that amounts to one corner of the table not having an adjustment valve. Three zones of control. Slave isolators are plumbed to one of the three outlets of the valve, so:
It is critical to note that there is ONLY one air supply port on these valves. It is marked with an arrow pointing away from it. For whatever reason, someone decided to install this pressure port pointed to an isolator on my table. That is wrong.
The top of the valve has a big rubber disc inside,. which has a brass part in the middle. The central brass part of the rubber disc bears has a hole in it. The brass part is what pushes directly on the valve actuator. The top “half” of the valve is the large (nameless in the first diagram) section with knurling. This holds the disc inside, and the Knurled Nut is the mounting hardware that holds the valve in place. The brass spacer floats loose in a hole in the top of the regulator and sits on top of the brass central part of the rubber disc. Hopefully your regulators are not too old and the discs are not too hard, so you don’t have to open them up for service. These should basically never have to be opened in normal service. If you do open a valve, make sure it gets reassembled with the disc in there or it will not work at all. Check out the Lever Arm:
There are bumps on the end. These push up to the Pivot Bracket. When the Table Top is too low, the Lever Arm will be pressed down, and the Lever will pivot down. The Arm Screw will then push on the Spacer. The Spacer pushes down on the diaphragm inside the valve. The diaphragm pushes down on the valve stem, opening the valve. Air will flow until the table rises enough to release pressure from the valve, and then the table will be at whatever height was set by the Isolator Height Adjust screw. If the table is too high, the pressure on the Lever Arm Screw is reduced, and the air pressure in the isolators will flow through a the hole in the brass part of the rubber disc inside the valve, up and out around the Brass Spacer. Air pressure is released until the table falls enough to press on the Spacer, cutting off the air supply.
This sounds maybe a little complicated but it works great, with an error band of about 0.050″ . . . the table height is controlled to a set level automatically. To adjust the table height, adjust the Isolator Height Adjust screw.
IF you followed all that, you will perhaps already know that it has to be plumbed correctly. If your air pressure supply is to one of the isolator piston outlets, when pressure is applied the valve will bleed pressure out the hole in the top (leaking by the Brass Spacer) as well as out the inlet of the valve. This means your isolator piston will always be pressurized to the same level as your air supply, and always sit at maximum height with no adjustment. Then it’s just an air spring, not an automatically-regulated adjustable height vibration isolator. I guess this is also an article to inform you how to convert your adjustable-height table into an air spring, but that would be silly for most cases.
A couple more pictures because i took them:
When the system is set up properly, the Arm is about horizontal, held in place and held at the proper angle by the Horizontal Arm Screw. Basically no air should be leaking out at this point. If the isolator lifts in this condition, your valve may have an old (hard) rubber disk inside. A temporary fix *might* be to unscrew the larger knurled section from the bottom of the valve body juuuust a little bit. This will mean a constant slow air leak, but it can be the difference between a table working and not working. It is also kinda fiddly to adjust, so make small adjustments, and be sure to wait a minute or two to allow the air to stop flowing between adjustments.
The slightest downward pressure on the end of the Horizontal Lever arm should cause the associated Isolator to immediately start filling with air and it should lift in a few seconds if there is no table holding it down.