Hydraulic flow control valves are not always visible

Saturday, November 3, 2012

Flow rate controllers are essential for safety. Look after them and they will serve you well

by RALPH WINFIELD

Hydraulic systems for power steering, lifting and many other functions have made life much simpler for most of us working with and around farm and construction machinery. Generally the systems are very reliable – if we take the necessary precautions to keep the fluid clean and cool.

However, sooner or later, most hydraulic systems have to be opened for repairs. Just recently, I had to remove the small, single-acting hydraulic cylinder that lifts my 60-inch mower deck. It had started to leak. The wiper on the shaft had obviously not been able to cope with all the dust encountered, as I do a great amount of mowing next to gravel roadsides.

Guess what I discovered? When I disconnected the hose from the 90-degree fitting at the cylinder, a small disc with a very small hole in the centre fell off the fitting. It was just a flow restrictor to control the speed of both the lift and lowering of the mower. In other words, it did not move. But please take heed. Many small and large orifice plates do move. If so, the direction or orientation of the orifice plate can be very critical.

Many flow rate controllers also move. Take, for example, the lifting and lowering of a relatively heavy boom or even a front-end loader on a tractor. The lift speed is often controlled by the flow rate capacity of the hydraulic pump. However, the return flow to an unpressurized fluid reservoir, possibly the transmission/differential housing, could allow the load to drop at a dangerously high speed if the flow is not controlled by a restrictor – usually a small orifice. If that is the case, the restrictor will be directionally sensitive. Thus it is absolutely critical that the flow rate controller be replaced in the correct direction.

The accompanying photo (page 66) is of a directionally sensitive flow rate controller. Fluid can flow around it in one direction, but only through the small orifice in the other direction. The restricted flow would be to control the drop speed of a heavy load.

As noted, this type of flow rate controller is essential for safety. However, whenever hydraulic fluid is forced to pass through small orifices, the heat created due to friction in the fluid is very significant. For most field-going (planting) equipment, the frequency of equipment drop is low enough that the fluid is not likely to become overheated.

If there is any risk of overheating, the equipment manufacturer will put fluid coolers in place. In most cases, there will likely be fluid-to-air coolers in front of the engine coolant radiator. If they exist on your equipment, do make a concerted effort to keep the cooler clear of dust, straw, etc. Nothing is more serious than overheated (oxidized) hydraulic fluid. It will cause excessive accumulations in the fluid filters.

Most industrial hydraulic systems have two items that we are not yet seeing in most agricultural applications. They are a hydraulic fluid temperature monitor and a pressure sensor for back pressure on the filter. Please remember that all hydraulic fluid is filtered just before being returned to the tank or reservoir.

Filter casings cannot tolerate high working fluid pressures if located in the working/load lines. If a filter other than a "pickup screen" is located in the pump intake system, a restricted filter will cause pump cavitation, which can spell disaster for the pump.

There are four things to remember, beyond checking hydraulic fluid levels regularly:

• Change fluid filters regularly.
• Make sure that all the wipers on all exposed hydraulic cylinder shafts are in good condition.
• If you take a hydraulic system apart at any location, look for flow restrictors and make sure they are replaced in the same direction.
• Cap or plug all opened hydraulic system fittings/hoses with plastic caps that are clean.
• Very slow hydraulic action usually indicates a worn pump, if fluid availability is not an issue.

However, many machines with one main hydraulic pump will often have at least one priority valve installed for steering. Please do not adjust or remove it! Your life might depend on its proper operation when the engine/pump is slowed down as you approach a ditch or a big tree.

Before working on a sophisticated hydraulic system, do obtain and fully understand a set of the schematics for the system.

Hydraulic systems are becoming more sophisticated every year. Look after them and they will serve you well. BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.