Intensifiers, also known as boosters, use a large quantity of low-pressure fluid to produce a smaller quantity of higher-pressure fluid. There are three classes of intensifiers: air-to-oil, oil-to-oil, and air-to-air.
Hydraulic boosters can develop and maintain high pressure for long periods of time without using power or generating heat in the circuit. They deliver fluid only when the cylinder demands it. Since all the oil from the booster is directed to the cylinder, there are no relief-valve losses.
Other advantages exist:
- Because heat is not generated while static hydraulic pressure is maintained and little is generated during rapid cycling, small oil reserves are required.
- Direction control of booster-operated cylinders is through air valves which are usually less expensive than hydraulic valves, but just as reliable. Pressure regulation of booster discharge is controlled by an economical air pressure-regulator valve.
- Because high hydraulic pressure is easily attained, booster-operated cylinders can be smaller in diameter. Booster systems are generally more compact than equivalent pump-and-tank units.
- Because pressure and direction-control valves are located in the air portion of the air-oil booster circuits, few fittings and usually no valves are required in the booster-to-cylinder connection.
- Air-to-air boosters permit the use of low-cost air cylinders at line pressures greater than those available from normal shop-air compressors.
Generally, boosters can be used as a hydraulic power source only in single-cylinder applications. A booster can drive more than one cylinder if the cylinders work in unison. However, to sequence two or more cylinders, additional boosters are required.
Boosters are limited in their volumetric capacity. For large quantities of high-pressure air or oil, large boosters are required. The double-pressure booster circuit can be used, but only if the driven cylinder load can be divided into a low-pressure traverse stroke and a reasonably short high-pressure stroke. If the high-pressure stroke must be long, and if the cycle must repeat rapidly, single-ram boosters offer little advantage.
Single-ram boosters are limited in their capacity to supply high-pressure oil. Therefore, only certain types of hydraulic valves can be used in air-to-oil booster circuits. Valves that have tank drainbacks should be avoided, because they use booster capacity during operation. Four-way hydraulic valves should also be avoided, because they generally cause momentary pressure drops in the system and throw the booster out of phase with the cylinder.
Because air is compressible, air-to-air boosters are somewhat less predictable than other types. However, if a stroke/diameter ratio of 3.7:1 is used to determine size of the output cylinder, calculated volumetric ratios of slave cylinder versus output cylinder should produce accurate results.
Selection of a booster type is probably the easiest decision of all. If a ready supply of shop air is available, an air-to-oil booster will be used. If low-pressure hydraulic fluid is readily available, an oil-to-oil booster is the best choice. If a one-shot source of fluid for clamping and holding is required, a normal one-shot booster will be used. However, if a continuous supply of fluid is required to reciprocate a cylinder or rotate a motor, a continuous booster should be considered.
Continuous boosters replace a pump in the system because they provide a steady output of high-pressure fluid. The two types are double acting and single acting. Both require valving or external controls to produce the necessary reciprocating action, but the single-acting model is generally simpler. Double-acting continuous boosters supply oil during both parts of the stroke. Therefore, output generally has less ripple.
Three major steps are involved in selecting the right booster for an application. First, booster size must be determined, making provisions for fluid compressibility. Next, the size of the makeup fluid tank must be determined, allowing for bleeding air from the fluid. Finally, output speed of the actuator must be checked to ensure that it is sufficient for the selected booster.
Booster size: If a cylinder requires high-pressure delivery throughout the stroke, a single-pressure booster circuit is needed. A double-headed booster can be used if the number of cycles per minute is low, and automatic bleeding and filling are not important. A triple-headed type must be used where rapid cycling is required. If the cylinder size is known, stroke, L, for a single-pressure booster can be calculated from
L = ( (Vc + Vo) / Ar ) + l
where Vc = total cylinder volume, Vo = oil volume loss due to compressibility, Ar = booster ram area, and l = booster ram pretravel.
If the maximum cylinder force is required only for the last portion of the stroke, a double-pressure booster can be used, with savings in booster size and air consumption. With such a booster, the normal circuit extends the actuator, and the booster supplies fluid only for the maximum force portion of the stroke. Booster stroke L for this arrangement is:
L = ( (Vp + Vo) / Ar ) + l
where Vp = volume required to move the cylinder through the high-pressure portion of the stroke.
Both equations contain a term for oil compressibility. In most hydraulic applications, oil is considered incompressible. But in high-pressure applications, it is safer to assume that oil can be compressed at the rate of approximately 1% per 1,000 psi.
Tank size: The air-oil tanks in booster circuits perform three general functions: 1. Make up for leakage. 2. Act as pressure sources to traverse or return cylinder. 3. Provide outlets for entrained air.
When the tank functions as a reservoir, its size depends on how much the system leaks. However, tanks are also outlets for entrained air. Here, the tank is not pressurized and acts primarily as a reservoir.
When functioning as a pressure source, tanks must have a volume slightly greater than the displaced volume of the cylinder. Volume of the tank should be enough to preclude oil level reaching the upper tank baffle at the high-level point. When at the low-level point, the lower tank baffle should not be exposed to air pressure. Pressurized tanks must also serve as an outlet for entrained air.
In rapid-cycle booster applications, oil has a tendency to churn as it flows back into the tank. This churning aerates the oil and produces excessive foaming. Each cycle then sprays foam out the air-valve exhaust. Well-designed tank baffles eliminate churning at oil velocities below 15 fps. A tank larger in diameter than the cylinder results in better surface quiescence during the fill stroke.
Tanks for rapid-cycle booster applications should have:
- Ports equal to or larger than the cylinder ports. Cycle rate should be set so that the flow through the interconnecting pipes does not exceed 15 fps.
- A diameter larger than the cylinder diameter. Tank height should be specified by matching rated-tank capacity with the cylinder displacement.
- Properly designed baffles.
- A location above cylinders and boosters so the system can be self-bleeding.
- A fill rate less than 4 fps.
Experts recommend that self-bleeding boosters be used wherever possible. This type of operation ensures that air does not build up in the system, causing "spongy" operation that impairs function and efficiency.
Air gets into the system in many ways, the most common being the use of two-position air valves. This results in high-pressure air being left "on" for long periods; the air dissolves quickly into the oil, causing spongy operation. This problem can be avoided by using a three-position, open-center valve and arranging the circuit so the tank can be exhausted by centering the valve, yet holding the booster "on." Another alternative is to use a bladder or piston separator in the air-oil tank.
In addition, air exists naturally in most hydraulic fluid, and vacuum in the system -- caused when boosters outspeed cylinders on reset -- can release air from the fluid. Also, the vacuum can draw air past seals into the system. Air tends to enter the fluid when it churns in the tank. These and other causes can rapidly aerate fluid in even a well-designed system, so the self-bleeding boosters are particularly important for applications that seem prone to aeration.
Cylinder speed: If rapid cylinder action is required, the hydraulic cylinder should be sized so that the reaction force (force required to do work) is 50 to 60% of available cylinder force at calculated pressure. Consider both high-pressure and low-pressure work reactions.
Air valves should be located close to the booster and air-oil tanks, and be connected with a minimum of piping. Three-way and four-way valves should be sized as though they were to operate a rapid-cycle air cylinder directly. The valves should be selected for low-pressure drop, fast response, and adequate internal area.
Air pressure-regulator valves, if used, should be selected for satisfactory flow capacity at regulated pressure, as well as matching port sizes. Self-relieving regulators prevent downstream pressure buildups. An air storage tank between the pressure regulator and the four-way valve optimizes air-valve response.
When estimating the speed of booster-operated cylinders, start by calculating oil velocity, Vo, in the lines leading to the cylinder:
Vo = ( 0.3202 * Q * Vp ) / Ap
where Q = cylinder displacement, Vp = cylinder piston speed, and Ap = internal area of pipe. For efficient flow conditions, oil velocity should not exceed 15 fps.
When air-oil tanks are traversing or resetting the cylinder, oil pressure is equal to air-line pressure -- usually around 80 psi. Pressure drop in a low-pressure circuit can materially reduce traverse speed. Sizing the cylinder so that the reaction force is approximately 50 to 60% of the available force at 80 psi usually offsets the effect of the pressure drop. On critical systems, special care must be taken to ensure that the system is piped with a minimum of fittings and coupled as short as possible.
If calculated oil velocity exceeds 15 fps, system line sizes should be increased to maintain speed. Cylinders and tanks also can be specified with larger-than-standard ports to minimize pressure drops.