A common rule of thumb for sizing a reservoir is that it should hold at least three times the rated flow of the pump. For instance, if your machine uses a 30 GPM pump, the reservoir should hold 90 gallons of oil. Typically, another ten percent is added to allow sufficient air space above the fluid to leave room for entrained air to exit the oil. For a lot of systems, this is a good design. Even under constant use, the oil gets to rest at least three minutes before cycling through the pump again. As long as ambient temperature is not unusually high, excess heat can be dissipated to atmosphere by radiation across the exterior surface area of the tank and enough volume is present so that high velocity oil from the return line comes to rest, allowing contaminants to settle and air bubbles in the oil to escape and ultimately exit through the breather.

This is a common design, but certainly not suitable for every conceivable system. Other factors must be taken into consideration. Many very large systems leave as much (or more) oil in the piping, manifolds and actuators as the reservoir holds. If the system drains when it is shut down, there must be room to hold this oil in the reservoir as well. Many systems use a check valve in the tank line to keep oil from draining back, but at some point components in the machine must be replaced, requiring at least a portion of the system to be drained. If the reservoir will not hold this additional oil, a procedure for collecting excess oil (without contaminating it) must be in place.

The bigger the system, the more we must take thermal expansion into consideration. Most mineral oil based industrial hydraulic fluids will expand approximately 0.06 cubic inches per cubic foot by volume for each Fahrenheit degree of temperature increase. In large systems, this can be substantial. More oil is required to extend a cylinder than to retract it. If your system has large or very many cylinders, oil level will change dramatically throughout a full machine cycle. Large accumulators may also affect oil levels. The last thing we would want is for the oil level to drop near or below the suction line of the pump and cause it to aerate.

While this all may suggest that bigger is better for reservoirs, the trend is actually toward making them smaller. Design objectives have changed over the years. Nowadays there are more requirements to save space, keep oil usage to a minimum and cut the overall cost of a system. Some measures can be taken to reduce the size of a reservoir. An efficient heat exchanger can eliminate the need for a very large reservoir, particularly when ambient temperatures are very high. Cooling a lot of oil in a high temperature environment by radiation alone can be impractical, particularly when considering the large amount of heat that can be generated by a hydraulic system. Even a very well designed machine that is properly adjusted and maintained can generate significant heat. Orifices, servo or proportional valves, pressure controls and even bends in the pipe - anything that causes the system to produce more power than is consumed by the load generates heat.

We determine reservoir size mostly by temperature. There must be enough surface area to dissipate heat to atmosphere if there is no heat exchanger. This is calculated by the following formula:

So, if we have 30 sq. ft. of surface area and a temperature difference of 50 degrees, we would calculate:

• In-plant Troubleshooting
• Leakage Problems
• Pressure Settings
• Shock Problems
• Hydraulic Reliability Assessments
• Hydraulic Troubleshooting Manual Development
• Startup Consulting and Recommendations
• Heat Problems
• Repeated Component Failures
• Speed Problems

1 2-pound piece boneless pork loin
1/2 teaspoon ground allspice
black pepper
1/2 cup dried cherries, chopped
1/2 cup fresh flat-leaf parsley, chopped
1 tablespoon whole-grain mustard
6 slices bacon
1 tablespoon currant jelly
1 teaspoon red wine vinegar

Heat oven to 350. Season the pork with the allspice and 1/2 teaspoon pepper and place on a rimmed baking sheet. In a small bowl, combine the cherries, parsley, and mustard. Spread evenly over the pork. Lay the bacon slices crosswise over the pork, overlapping them slightly and tucking the ends underneath. Roast for 45 minutes. In a small bowl, combine the jelly and vinegar. Brush over the bacon and continue roasting until bacon is crispy, about 10 to 15 minutes more. Let rest at least 10 minutes and slice into half-inch slices.

Operators and technicians alike, anyone who works on or around a machine with an accumulator should see this DVD. It could make the difference between working safely and serious injury or DEATH!