Two basic goals should be kept in mind when replacing hydraulic hose assemblies on crane equipment: make the system as leakproof as possible; and make the system accessible for ease of maintenance.
Most hydraulically operated equipment will come from the factory of the original equipment manufacturer (OEM) with the hoses routed properly. That is, the hoses run parallel on the piece of equipment, follow its contours, and have the proper attachments. They also have a proper layout with regard to the location of port positions, valves, filters, heat exchangers and the fluid reservoir.
If, however, the hose has been previously replaced, equipment maintenance personnel should pay close attention to routing the new hose installation. Never assume the old hose was routed properly.
Here are some recommendations to consider when replacing hydraulic hose assemblies.
Hose ID Plan the routing of the largest internal diameter (ID) lines first. This is because they are the hardest to bend and manoeuvre, especially in tight spaces. Smaller ID hoses can then be routed in tighter areas. This makes future changes or additions easier, more convenient and more economical.
Hose length Select the proper length of hydraulic hose. Excessive lengths increase the opportunity for pressure drop, heat build-up and likelihood that the hose will be damaged by external obstructions. Hoses that are too short are liable to cause failure at the fittings. Remember, hydraulic hoses can elongate and contract by up to 4% during pressurisation cycles. Another solution is to select a replacement hose that is rated at half the Society of Automotive Engineers (SAE) bend radius specification. These hoses are extremely flexible, easy to install and can be used in shorter lengths. Some can reduce hose costs by as much as 40%.
Flexing The key point is that hydraulic hose is intended to flex, not twist. Laboratory tests have shown that if a large ID, high pressure hydraulic hose is twisted only 7°, service life can be reduced by 90%. Also, high pressure hose should not be routed through several bends. If the system dictates such a situation, separate the hose into two or more sections so each will flex through just one plane.
Pivots When the hose must flex, be sure to route it through the pivot point around which the component is moving. This will result in the best and most efficient flexing of the hose line, will use the shortest length of hose, and will keep the hose within the contour of the machine. This can be achieved by bending the hose like a hinge. Otherwise, the hose may have a tendency to take an S-bend, which is most likely when the hose is pushed rather than bent. An S-bend installation results in excessive hose movement and diminished service life. In routing a hydraulic hose through a pivot point, consider the relative positioning of the two end fittings to avoid the S-bend. Extend the moving component to its farthest point where the maximum hose length will be required. Then place the fittings in parallel planes. The hose should flex in a hinge-like fashion when the component is swung back to the starting point.
Reciprocating motion In addition to flexing, the end of the hose may have to move back and forth. The following approaches can solve this.
1) Rolling – The hose is arranged in an unbalanced U-shape, one upright of which is stationary and longer than the second. The second leg is free to reciprocate parallel to the first.
2) Hose reels – These are for use with high pressure hose, and are equipped with high pressure swivel joints and a spring return to help rewind the hose.
3) Hanging – Hose is hung in loops from a support. As one end of the loop moves away from the other, the loops are able to adjust to the changing centre-to-centre distance.
Rotary motion Rotary joints are commonly used to provide rotary motion. If there is a rotary motion, a rotary joint will be necessary.
Fire Protection If the fluid is not phosphate ester based, and the hose must be routed close to hot, hazardous components such as exhaust manifolds, the hydraulic hose must be protected. This can be accomplished by:
1) Routing the hose through a steel tube or channel.
2) Covering the hose with a larger ID outer hose.
3) Using fire sleeves over the hose.
4) Anchoring the hose away from the extreme heat source.
Abrasion protection A primary source of hydraulic hose failure is abrasion resulting from cuts, friction caused by other moving hoses, or objects in the operating environment. Hose-to-hose abrasions are likely to show up where a length of hose travels through a boom or bulkhead, or along framework.
Erosion of the cover material can also be caused by non-compatible fluids such as toxic chemicals, acids, detergents and non-compatible hydraulic fluids. When exposed to air, the hose reinforcement is subject to rust and accelerated damage, leading to failure.
In most cases, abrasion problems can be spotted during routine inspections and replacement can be initiated before a major failure occurs. Bundling together hoses that flex in the same direction can help solve hose-to-hose abrasion. Clamps, bent tube couplings, nylon ties, clamp collars, spring guards and sleeving can be used to keep hose away from the source of abrasion or exposure to non-compatible fluids.
Protect the hose cover by using nylon and urethane sleeving and spring guards. Abrasion-resistant hybrid hose with far greater abrasion resistance than any standard hose is also available to solve abrasion problems. Made of special rubber/plastic compounds, the new cover materials have been tested to last up to 300 times longer than standard rubber covered hoses. When installed, they increase service life, lower maintenance, and eliminate the need for costly hose protectors such as guards, sleeves and bundling.
Clamping In cases where clamping is necessary to properly route the hose, choose good quality clamps capable of taking considerable stress for high pressure lines. The ID of the clamp should be 1/32 inch (0.8mm) smaller than the hose outside diameter (OD) to provide a positive, non-abrading clamping action. Anticipate hose expansion and contraction of ±4%. Avoid clamping hose to vibrating components. And never clamp a hose on its bending arc.
Avoid crossing hose lines. When crossing lines is unavoidable, join the two lines at the junction point. Never clamp parallel high and low pressure hydraulic hoses together – the differential change in hose length during pressure cycles will result in a seesaw action that will damage both hoses.
Careful consideration of the placement of hydraulic hose circuits can help ensure maximum equipment efficiency, minimum downtime and simplify maintenance procedures.
About the author
Todd Mueller is manager of the hydraulic hose/connector product application department of The Gates Rubber Company, based in Denver, Colorado, USA
