Treat your hydraulic hoses with the respect they deserve

Sunday, March 8, 2015

Hoses are integral parts of a hydraulic system, but correct routing, physical protection and proper installation are essential if they are to have a long service life

by RALPH WINFIELD

Just recently, I wrote about keeping newer hydraulic systems clean. But it's just as important to protect the hydraulic hoses that are integral parts of working machines like combines or send hydraulic fluid to remote pieces of equipment for a variety of functions.

While it is often said that the hydraulic pump is the heart of the hydraulic system, we must give due consideration to the other system components as well. Hydraulic hoses are commonly integral parts of a hydraulic system. They are often used to provide flexibility to the system. In other words, they allow for movement of hydraulically-operated components.

When hoses are properly selected for temperature and operating pressure, they should provide a long service life. Note I say should. Routing, physical protection and proper installation are three very important factors in determining the service life of hydraulic hoses.

Hydraulic hose design. All high-pressure hydraulic hoses are made up of three layers. The first is the inner tube, which is a smooth synthetic rubber layer that is oil-resistant, flexible and able to resist heat and corrosion. It must carry the oil without leakage.

The second or middle layer is the reinforcement layer. For high pressure oil, this layer primarily consists of braided wires. The strength of this layer normally determines the operating or working pressure of the hydraulic system. You will note, if you look at the cut end of a new hose, that the wires run diagonally or in a spiral pattern around the hose. This is very important to you as a user. The reason: when the hose is pressurized, its diameter increases. Simultaneously, its length decreases because of the hose expansion. The visible result is "hose squirm" as the hose responds to pressure/no pressure working cycles.

The third layer is the outer cover. This special rubber protects the reinforcement layer(s) from abrasion and exposure to weather, oil, and dirt – in other words, anything that would destroy the integrity of those very important braided wires.

Protecting hoses. Hydraulic hoses allow for functional movement and effectively act as mini-accumulators as they clip pressure spikes. They do this by absorbing these short-term pressure spikes, thus eliminating the effect often called "water hammer." Therefore, hoses must be free to move. However, that squirm movement can cause significant damage to the outer (protective) cover if the hose rubs against any metal components or even another hose.

It is critical that you observe hydraulic hoses for the first signs of abrasion of the outer cover. In some instances, the hoses can be moved in clamps or protected where they contact a prominent metal object. These actions can and must be taken, even on new machines. Hose routing can be a problem at the manufacturing level because of different equipment options available. Do not assume that the manufacturer or assembler gets the routing perfectly every time.

Other deadly sins. When installing new (replacement) hoses, here are a few things to be aware of:

1. Never install a hose so that it has a twist along its length. Use two wrenches when necessary – depending on the type of fittings – to hold the end of the hose so that it is not turned during tightening of the second connector. A twisted hose will cause the hose layers to separate when the hose is subjected to normal working conditions (pressure cycles) that cause it to squirm. The worst-case example is the separation of the inner tube or layer in a supply hose to a pump. It will collapse and starve the pump possibly causing cavitation, which will destroy the pump if not detected early. But remember that the hose may look just fine from the outside!

2. Do not otherwise restrict the supply of oil to a hydraulic pump. Never put a filter between a hydraulic tank and the pump. Strainers are OK here as they just keep large particles out of the pump. Oil added to the tank and returning to it must be clean! A partially plugged filter ahead of the pump will also cause the inner hose layer to separate and collapse. This is another cause of catastrophic hydraulic system failures!

3. Never install a hose that only just reaches. A taut hose will pull loose from the crimped connector at one end as a result of pressure cycles. Oil will start to leak between the hose layers or externally. An external leak is easy to find, as the hose end will be fluid-covered. If it just leaks internally, the outer cover may start to look like the cheeks of a chipmunk.

4. Try not to use hoses that are excessively long. The resultant hose loops can cause interference, but are more likely to kink – again causing layer separation. Small pieces of hose liner can cause all kinds of problems in hydraulic systems. The common ones are sticking control valves, inoperative selector controls or plugged (inoperative) restrictor (flow control) valves.

5. Protect hoses from pinch locations or hot spots. Resultant hose failure will be sudden and early. Provide protective shielding if necessary to keep hoses from these hazards.

6. Hoses to remote cylinders and motors on equipment powered by tractors are often at risk. When you connect that piece of equipment the first time, be it to a plow or a planter, do look very carefully and observe the hose movements when the tractor makes U-turns and/or the hydraulically powered equipment is operated.

Always remember to treat hydraulic systems and hoses with the respect they deserve. If you suspect a high-pressure leak from a hydraulic hose, do not use your hand to try and locate the leak. Oil under pressure will penetrate the skin and cause blood poisoning. If it does happen, get medical attention immediately.

Whenever you must open a hydraulic system to repair or replace a component such as a hose, do make every effort to keep dirt out of the open connections and the new component. Do not over-tighten NPT and JIC hose fittings, as they will create metallic particles within the system. (Many original equipment manufacturers have switched to 0-ring straight thread and flanged fittings to reduce the risk of connector contamination.)

After a hydraulic hose or other component has been replaced, the reservoir (tank) has been topped up and the system operated for a short time to check for leaks, go that final mile. Replace the hydraulic system filter. Filters are a lot cheaper than almost all other hydraulic system components. If the filter becomes loaded with contaminants, the hydraulic fluid will bypass the filter. The bypass valve, usually in the filter head, is there to prevent the filter from bursting and dumping all the collected contaminants into the system.

The filter bypass valve is also necessary to permit hydraulic fluid to bypass the filter during cold weather start-ups when the fluid is very viscous. Thus, a light duty warm-up period is essential. The normal bypass pressure is about 25 psi (170 kPa). Some electronic dash panels show a red "idiot" light to indicate when the bypass valve is open.

Some newer self-propelled field sprayers have a prominently located small pressure gauge at the filter head. The gauge might just have coloured sections on the dial face – the same idea that we have seen on pre-charged fire extinguishers for many years.

Pressure gauge or "idiot light" – they are both excellent reminders that some action is necessary now. BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.