Figure 1. Double-acting pneumatic cylinder in single-pressure circuit.
Figure 2. Response time in seconds for single-pressure circuit.
Figure 3. Double-acting pneumatic cylinder in dual-pressure circuit.
Figure 5. Response time in seconds for dual-pressure circuit with only retract pressure reduced.
Figure 4. Response time in seconds for dual-pressure circuit with extend and retract pressures optimized.
By Dale L. Kohlsmith
Director of Training
Prior to 1949, pneumatic valves that operated double-acting cylinders were typically 4-ported, 4-way valves with one port for pressure, one port for exhaust, and two ports piped to the cylinder — similar to today's hydraulic valves. However, with the invention of the spool-and-sleeve valve by Numatics Inc. and the industry's subsequent development of similar designs (mostly rubber-packed, spool-type valves), 5-ported, 4-way pneumatic valves have become the standard in the 21st century.
Despite the fact that these modern valves open the door to more sophisticated circuits, most applications still are plumbed as singlepressure applications, with a single supply pressure and dual exhaust approach. Although this arrangement provides an optimal exhaust condition and allows a faster response time than the historical 4-ported, 4-way, single-exhaust valve, it is still not the best-case piping for most applications.
Most pneumatic double-acting cylinder applications do not require the same force in both the extend and retract conditions. For example: a clamp only needs high force to clamp a part, while resetting the clamp does not; a conveyor stop only needs relatively high force to stop a part on a conveyor, not to release the part; a punch only needs high force to drive through the material, not to reset.
Hundreds of other similar examples exist, thich is part of the reason why double-acting cylinders are so popular. They inherently generate a higher extension force (compared to their retraction force) because of the difference in surface areas on the piston faces. This is Pascal's Law (F = PA) in action.
Most often, the pressure and the bore of the cylinder are selected to provide the highest force required by the application in the extend direction. The valve is then piped up to the cylinder using a single pressure supply with the knowledge that there is enough force to extend and more than enough to retract. Tradition says, "That's good enough," if you have deep pockets to pay for inefficiency.
Incorporating dual pressure into most of these systems will improve energy efficiency. The concept of dual pressure is not new. In fact, more than 50 years of history prove that it can save both time and money. It is true that an additional regulator must be added to gain the second pressure, but this only amounts to a few dollars extra initial cost per circuit. Why, then, do so many applications still use only single pressure, when dual pressure pays for itself in months — either through reduced air consumption or increased production?
In any double-acting cylinder, the total load is a combination of three factors:
- the external load
- the internal friction load at the piston and rod seals, and
- the exhaust backpressure preload.
The exhaust preload is the resistance due to the air pressure on the rod face of the piston when extending a cylinder. While a cylinder is retracted, there generally is some pressure to hold it in position. When the valve shifts, air pressure pushes on the blind side of the piston, causing the cylinder to start to extend. However, if the rod side of the cylinder is still pressurized, the cylinder will stroke slowly, or perhaps not at all. That is why 4-way valves are so popular. They conduct pressure to one side of the piston and exhaust the other simultaneously. Supply pressure pushes in, while exhaust pressure is pushed out. The resistance of the air being pushed out is the exhaust preload. (The same happens in reverse to retract the cylinder, because air pressure is holding the cylinder extended until the valve shifts back to its original position.) Note that of the three load factors, exhaust preload is the only one the designer can control — and he or she can do that with a dual-pressure circuit.
Let's look at an example: The external load on the extend direction of a cylinder is 60 lb and the external load on the retract direction is 10 lb. If the available air pressure 75 psig, then a relatively small cylinder could do the job. Most applications are still piped for single pressure, so we will start with the circuit in Figure 1.
The total response time of a cylinder is the combined time of the coil/valve response, the time delay due to the exhaust preload in the circuit, and the delay due to static friction of the cylinder seals. (Note, if a dynamic-seal valve were used instead of a spool-and-sleeve style construction, the static friction of the valve seals would also add a time delay.) For this example, response time is 0.40 seconds for the cylinder to extend, as shown in Figure 2. A similar curve would exist for the response time for the cylinder to retract.
This is the typical scenario for most pneumatic applications where the same pressure is used to extend and retract the cylinder, even though the reset load is usually substantially less than the work load. If the exhaust preload can be reduced, less extension force will be required and speed will increase,-or response time will stay the same with less air used. By simply optimizing the pressures on the same size cylinder, pressure in both the extend and retract directions could be reduced and still achieve the same response time, because the internal load, due to exhaust preload, would be reduced. Although it is true that higher supply pressure means more force, there is a misconception that more air generates more speed. This is not always the case, as it is equally true that more pressure into a cylinder means more pressure must be exhausted from the cylinder.
Now let's repipe the cylinder in the example for dual-pressure operation, Figure 3. By reducing the pressure in both the extend and retract directions, the exhaust preload in both directions is also reduced. Therefore less force is required to move the cylinder in either direction while speed can be maintained, Figure 4. In this circuit, the primary benefit is air conservation. By reducing the air pressure from 75 psig in both directions to 49 psig on the extend and 21 psig on the retract, an almost 50% air savings could be achieved, thereby reducing overall production cost and making the manufacturing operation more efficient.
Another dual-pressure arrangement would increase speed while increasing efficiency to a lesser degree, Figure 5. Here the extend pressure is maintained at 75 psig while the retract pressure is set at 20 psig. The example work stroke response time is decreased from 0.40 sec to 0.28 sec and an almost 40% air savings could still be realized. Note that the reset stroke could be marginally increased in this case, but often the reset is not as critical as the work stroke.
As stated earlier, dual pressure is not a new concept by any means, but it is one simple step toward making pneumatic systems more efficient in a very competitive world economy.
Dale L. Kohlsmith can be reached at [email protected].