A suction strainer (as shown at the lower left in Figure 7-8) often is found on the pump inlet line. Strainer is a common term for filters with openings of 75 m1 or larger. Strainers on the pump inlet line protect the pump from large, damaging contaminant particles that could cause catastrophic failure. These particles might be startup debris left in the tank and piping or large contamination introduced to the system from external sources or from internal part failure.

Pumps without supercharged inlets can only tolerate a portion of one atmosphere pressure drop without affecting inlet flow. With this low pressure drop, (14.7 psi maximum at sea level on an average day), a restriction such as a low-micron filter can cause the pump to cavitate. Cavitation causes pump failure faster than dirty oil; so avoid it in every situation. (See the write up on pump inlet conditions and cavitation in Chapter 8.)

Suction strainers are available with 75- to 150-µ openings. Some manufacturers have inlet filters with ratings as low as 25µ. A low-micron element needs large filtering surfaces to keep pressure drop low. When a pump is force fed by another pump -- sometimes called a supercharging pump -- a low-micron rated element can be used. The supercharging pump forces fluid through a very fine filter to the working pump, thus keeping it from cavitating.

A suction strainer or filter should have a bypass relief valve. Set the bypass to open at a pressure of 1 to 3 psi if the strainer clogs. The reasoning behind this is that the pump will run many hours on contaminated oil, but will fail in a short time with little or no oil. Suction strainers may be located inside or outside the reservoir. Internal strainers are less expensive, but their condition is more difficult to monitor. External strainers are easy to service and often have an indicator to show when the filter starts bypassing. The indicator can be as simple as a vacuum gauge or it might be a vacuum-operated electrical output to a warning light, alarm, or shut-down controller.

Many older circuits have nothing but a suction strainer for filtration. Retrofitting these systems with off-line or kidney-loop filters (discussed later) is advisable.

Return-line filters

Another common location for filters is in the return line. (Figure 7-8 indicates this location at the right center.). A return-line filter keeps most contamination that is caused by part wear or ingestion from getting into the tank, this protecting the whole system. Return-line filter protection ratings typically range from 3 to 25 µ. Obviously, you should select a return-line filter rated for the desired system-cleanliness level or less.

Return-line filters should have integral bypass check valves. If the filter becomes loaded, return oil needs an open flow path to tank until it is convenient to change the filter. Without a bypass, the filter element probably will collapse, or the element housing or seal may rupture.

Typical bypass checks require 10 to 50 psi to open. The bypass pressure should be high enough to stop fluid from going around the filter during normal conditions, but low enough to avoid damaging filter element and its housing seal.

Some designers size return-line filters just large enough to handle the pump’s rated flow. This can cause problems, especially if cylinders in the circuit have oversized rods, or if one cylinder must return one or more other cylinders. For example, if a cylinder has a 2:1 rod diameter, flow to tank while the cylinder is retracting is double the pump flow. Sizing the filter just for pump flow in this case allows contaminated oil to bypass the filter, and may damage the housing or seals. Paper filters can collapse, have holes blown through the element, stop filtering, and never indicate they need replaced. On pleated elements, the pleats can collapse, giving a “loaded element” indication prematurely.

Even with a correctly sized return-line filter, flow through it changes constantly. Steady flow through the element gives the most efficient filtering. If a filter passes constant flow, the bypass valve will not open until the filter fills with contaminants. This means only clean fluid leaves the filter. Visual and electrical indicators are available to show when a return filter is bypassing.

Pressure filters

Another location for filters is in the pressure line (as shown at the left middle of Figure 7-8). These filters are mandatory in systems using servovalves. Servovalves have low contamination tolerance. They have small internal orifices, very close tolerance fits, and must shift rapidly at low pilot pressure differential. A servovalve can stop functioning in as little as two minutes when supplied with oil that is clean enough for a typical hydraulic system. Even when a 3-µ return-line filter is in place, contamination generated by the pump is enough to shut down a servovalve in a short time. (Note that fluid for a proportional-valve circuit often requires the same cleanliness level as a servovalve circuit to maintain fast response and consistent operation.)

Actually, pressure-line filters would be an added advantage for any hydraulic circuit, but high initial and replacement cost limits their use. Pressure-line filter housings must be strong enough to withstand full system pressure. When there is a high pressure drop across the filter, the element must not collapse. These requirements make filter housings and elements much more expensive than other type filters. Pressure-line filters usually have elements with 1- to 5-µ openings. The pressure-line filter should have an absolute rating, or have a Beta Ratio of 50 or higher. A pressure-line filter should not have a bypass. If the filter element clogs, it is better to stop flow to a servovalve than to contaminate it. Visual and electrical clogging indicators are available for most pressure-line filters. They warn of potential clogging so that elements can be replaced well before production speed is affected.