Machinery Lubrication

Machinery Lubrication March April 2018

Machinery Lubrication magazine published by Noria Corporation

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14 | March - April 2018 | www . Check va lves are the simplest form of hydraulic devices in t hat t hey permit free oil flow in one direction and block oil flow in the opposite direction. Check valves may also be used as a directional or pres- sure control in a hydraulic system. In Figure 1, oil is flowing in from the left side port, through the check valve and out the right side port. If the pressure equalizes or is higher in the right side port, the check valve will close and block f low in the opposite direction. e spring rating varies based on how the valve is used in the system. One of the most common locations for a check valve is immediately downstream of the hydraulic pump (Figure 2). Notice that no spring is shown with the check valve symbol. When used in this application, the spring pressure rating is usually 1-5 pounds per square inch (psi) and therefore not shown with the symbol. In this case, the valve is used as a directional control in that it allows oil flow from the pump to the system but blocks flow in the reverse direction. is is commonly called a pump isolation check valve. is valve serves four purposes within the system, which are detailed below: Block Pressure Spikes The check valve will block pressure spikes back to the pump. Depending on the pressure, oil flows from the pump to the system at a speed of 15-30 feet per second. When a directional is de-energized to block flow or a cylinder fully strokes, the oil is rapidly deadheaded. e pres- sure in the line can quickly increase by two to three times. e check valve should then close and block the pressure spikes to the pump. I recall a plywood plant changing four pumps due to cracking of the pumps' housings. This occurred over a week's time on the debarker hydraulics. When the plant ran out of pumps, the staff finally took out the check valve and found that the piston and spring were no longer in the valve. is $150 check valve cost the company $15,000 in replacement pumps and another $50,000 in machine downtime. at was one expensive check valve. e truth is that if one mechanic had looked at the schematic and known why the check valve was in the system, the replace- ment of the pumps and subsequent expenses would have been avoided. Prevent Oil Lines from Draining When a system is shut down, it is important to maintain oil in the The Importance of Check Valves in Hydraulic Systems HYDRAULICS Al Smiley | GPM Hydraulic Consulting Figure 1. In this system, oil flows from the left side port, through the check valve and out the right side port. Figure 2. Check valves are often located immediately downstream of the hydraulic pump.

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