Hydraulic cylinder rod travel is slower than normal – do I find the problem with a pressure gauge or flow meter?

The single, most common problem with a hydraulic system is that the actuator (cylinder or hydraulic motor) slows down progressively as the oil temperature increases. There seems to be some confusion as to whether the problem is related to pressure, or flow, so let’s clear the matter up.

If the hydraulic cylinder holds 5 gallons (18.75 liters) of oil when the cylinder rod is fully extended, and you want the cylinder rod to extend from the fully retracted position to the fully extended position in one minute, how much oil will the hydraulic pump have to pump into the cylinder to extend the rod in one minute? That’s right – 5 Gallons Per one Minute, or 5 GPM (18.75 Lpm) [See Figure 1].

Now, if the cylinder extends in two minutes instead of one minute, is the problem caused by a decrease in flow, or a decrease in pressure? Let’s figure it out! If the cylinder holds 5 gallons (18.75 liters) of oil when the cylinder rod is fully extended, and the cylinder rod extends in two minutes, that simply means that instead of pumping five gallons into the cylinder in one minute, the cylinder is only receiving 2.5-gallons (8.45 liters) in one minute [See Figure 2]. That means that the pump flow has decreased from 5 GPM (18.75 Lpm) down to 2.5 GPM (8.45 Lpm), or 50%. Clearly, the only reason why the cylinder rod speed decreased is because the pump flow decreased, which simply means that any time the speed of a cylinder decreases, the problem MUST be a decrease in flow, and NOT a decrease in pressure.

The point is, whenever a machine operator complains that a hydraulic cylinder, or hydraulic motor, slows down, what the operator is saying is that the cylinder isn’t receiving enough oil. There is only one instrument that can be used to find the problem, and that is a flow meter.

Bear in mind that it is virtually impossible for a pressure decrease to cause flow to decrease, therefore a pressure gauge cannot be used to detect a problem related to decreased actuator speed. Likewise, it is impossible for an increase in pressure to cause the speed of an actuator to increase.