The amazing quick exhaust valve
Several years ago, a colleague insisted that the cure to most pneumatic system ailments was to "hang a quick exhaust on it!" Although a little over-simplistic, the suggestion was not far from the trut...
December 1, 2001 | By Ted Grove
Several years ago, a colleague insisted that the cure to most pneumatic system ailments was to “hang a quick exhaust on it!” Although a little over-simplistic, the suggestion was not far from the truth. The quick exhaust valve has many uses other than the one implied by its name. A key one is cylinder speed control.
Slow or sluggish cylinder speed is most probably caused by restriction or pressure losses in the exhausting lines of the cylinder/valve piping system. The normal practice of using adequately sized piping and locating the valve as close to the cylinder as possible should be followed, but when all else fails, “hanging a quick exhaust” in the exhaust line as close as possible to the cylinder should help.
The effect, of course, is to dump the exhausting air directly to the atmosphere at the cylinder, eliminating the restrictions caused by the exhaust network (Fig. 1).
If a four-way valve controlling a small cylinder is mounted a relatively long distance from the cylinder, lubrication may never reach the cylinder. The volume of the lines leading to the cylinder may be greater than the swept volume of the cylinder. In this case the lubricated air would not get out of the line to the cylinder and would be exhausted from the line on the return stroke.
The quick exhaust valve, when placed close to the cylinder, will of course dump the exhausting air directly to the atmosphere and leave the lubricated air in the line, available to be fed into the cylinder on the next stroke A flow control valve may still be used to control the speed of the cylinder if it is placed between the quick exhaust and the cylinder.
A look at a cutaway of the valve (Fig. 2) and a fluid power symbol (Fig. 3), which shows its operation, should help us to understand some of its uses. As you can see, the only moving part in the valve is a floating, flexible sealing disc that is biased to cover the exhaust port but flexes to seal either the pressure or exhaust port, depending on port pressures or combinations of pressures. Fig. 3 shows the valve piped as a quick exhaust with Port A connected to the valve, Port B to the cylinder and Port C open to the atmosphere.
The floating disc type of quick exhaust may also be used as a shuttle valve to allow control or pilot signals to be received, or cylinders actuated, from more than one source (Fig. 4). Without the double checking action of the shuttle valve, the air from each source would be vented from the exhaust port of the other source. Two check valves would not work because exhausting could not take place.
By plugging Port 3 (the exhaust port in Fig. 2), this handy valve becomes a check valve with a very low cracking pressure, or pressure required to move the flapper off the seat, and the capacity to handle relatively large flows. It is suitable not only for normal compressed air systems but also for vacuum systems. Remember, however, that the flow of air is reversed in vacuum systems, which causes some confusion in assessing which direction the check valve should be installed.
Cylinders, when used as air balancers or air springs (Fig. 5) powered by a constant regulated air pressure, have the advantage of exerting the same force over their entire stroke, something that a spring can’t do. This system is excellent for balancing heavy loads such as manually operated tooling used on conveyor-type automated assembley systems.
A problem occurs with most relieving-type regulators as the cylinder extends. The flow through the relieving port is too small to allow an adequate speed of extension. If you “hang a quick exhaust” between the regulator and the cylinder, the problem is solved.
It is interesting to note that the quick exhaust will shift, closing the exhaust port, the instant that the pressure in the cylinder is less than the pressure in the regulator. In other words, the quick exhaust valve strives to keep the cylinder pressure exactly the same as the regulated inlet pressure.
The phenomenon is very useful in truck air brake systems where the brake pressure must be applied and released rapidly or feathered for a more sensitive braking action. The diaphragm braking cylinders are sometimes quite a distance from the control valves, making the quick exhaust doubly effective.
Ted Grove is a widely experienced fluid power trainer and is corporate training manager of Wainbee Limited of Mississauga, Ont. He can be reached at firstname.lastname@example.org. Previous columns can be viewed at www.mro-esource.com.