Machinery Lubrication

Machinery Lubrication September October

Machinery Lubrication magazine published by Noria Corporation

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Page 18 of 69

14 | September - October 2016 | 14 | September - October 2016 | HYDRAULICS Temperatures Controlling Hydraulic Oil By Al Smile y, GPm HydrAulic conSultinG With cooler weather on the way, you may not be too worried about rising oil temperatures, but the fact is, any indus- trial hydraulic system running higher than 140 degrees is too hot. Consider that for ever y 18-degree increase in temperature above 140 degrees, the life of the oil is cut in half. Systems that operate at high temperatures can produce sludge and varnish, which result in the sticking of valve spools. Pumps and hydraulic motors bypass more oil at high temperatures, causing the machine to operate at a slower speed. In some cases, high oil temperatures can waste elec - trical energy by making the pump drive motor pull more current to operate the system. O-rings also harden at higher temperatures, leading to more leaks in the system. So what checks and tests should you perform if the oil temperature is higher than 140 degrees? Causes of Heat Generation Ever y hydraulic system generates a certain amount of heat. Approximately 25 percent of the input electrical horse- power will be used to overcome heat losses in the system. Whenever oil is ported back to the reser voir and no useful work is done, heat will be generated. The tolerances inside pumps and valves are normally in the ten-thou- sandths of an inch. These tolerances permit a small amount of oil to continu- ously bypass the internal components, causing the f luid temperature to rise. When oil is f lowing through the lines, a series of resistances will be encountered. For example, f low controls, proportional valves and ser vo valves control the oil's f low rate by restricting f low. When oil f lows through the valves, a "pressure drop" occurs. This means that a higher pressure will exist at the valve's inlet port than the outlet port. Anytime oil f lows from a higher pressure to a lower pres- sure, heat is generated and absorbed in the oil. When a system is initially designed, the reser voir and heat exchangers are sized to remove the generated heat. The reser voir allows some of the heat to dissi- pate through the walls to the atmosphere. If properly sized, heat exchangers should remove the balance of the heat, enabling the system to operate at approximately 120 degrees F. Pressure-compensating Pumps The most common type of pump is the pressure-compensating, piston-type pump. The tolerances between the pistons and barrel are approximately 0.0004 inch (Figure 1). A small amount of

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