Pumps: The "muscle" of a fluid-power system; converting mechanical energy to fluid form.

There are six basic considerations in evaluating and selecting a hydraulic pump. They include these points:

• Pressure rating. Pressure rating is based on the limitations of the pump from damage and leakage due to excessive pressure as well as the limitations of downstream components; it is a major consideration in determining if the pump can do the intended job.
  Pumps are designed to withstand ranges of pressure from 50 to 15,000 pounds per square inch (psi), but the majority will be in the 2,000 to 4,000 psi range. Vane pumps are usually from 2,000 to 4,000 psi. Internal-gear units have lower pressure ratings, they usually do not exceed 3,000 psi. Most piston pumps are designed for a maximum rating of 3,000 psi, although some are designed for 5,000-6,000 psi. Determining the pressure necessary for a particular operation often dictates which pump should be chosen.
  
• Flow rating. This is the second most important consideration in selection of a pump. Flow is expressed as a volume of output in gallons per minute (gpm). Flow rating is based on performance under a specific set of control conditions - for example: Vickers vane pumps for gpm are rated at 1200 rpm. The manufacturer will usually state in their literature the conditions under which the rating is made.
  
• Speed rating is a third consideration, which may be limited by the ability of the pump to fill without cavitating or other mechanical considerations. Permissible speed ranges and inlet pressure requirements for a specific pump design are also usually stated in the manufacturer's literature.
  Efficiency is an indication of the quality if the pump. The importance of this calculation is to select a pump with a slightly higher rating than the theoretical need. Three ratings are usually used.
  
• Volumetric efficiency: The ratio of actual to theoretical delivery. The difference is usually due to internal leakage necessary to lubricate the pump. Volumetric efficiency is commonly in the mid to high 90s.
  
• Mechanical efficiency is the ratio of overall efficiency to volumetric efficiency. Mechanical losses are usually attributed to fluid compression and internal friction.
  
• Overall efficiency is the ratio of hydraulic power output to mechanical power input. Fluid compatibility is an issue of growing concern. Petroleum oil is typically the hydraulic fluid used in most applications. Environmental and safety concerns are encouraging increasing acceptance of water-based and other environmentally acceptable fluids. Most pumps have been developed for petroleum-based fluids.

Use of non-petroleum fluids often causes problems. Frequently speed and pressure ratings have to be reduced and different seals are necessary, therefore a special pump needs to be selected to operate with these fluids. Most manufacturers can advise you about pump selection when nonstandard fluids are to be used.

Size and weight can be straightforward means of comparing pumps. These comparisons are described as a power/weight ratio. Some common mobile types have a ratio of around .75 hp/pound. The axial-piston type that is widely used in industrial, marine and aircraft applications may vary from that ratio to around 2.5 hp/lb, while a highly-refined (costly) piston type pump capable of delivering 4 hp/lb, may well be justified for use in aircraft.
With these factors in mind, one can make knowledgeable decisions in the selection of system components.

In our next issue, we will describe gear pumps and their variations. Gear pumps are probably the most common pumps of all. Your automobile oil pump is probably a gear pump.