Hydraulic Units and General Maintenace

Hydraulic reservoirs

Hydraulic systems require a finite amount of liquid fluid that must be stored for reusing continually as the circuit works. Hence, part of any hydraulic circuit is a storage reservoir or tank. This tank may be either a part of the machine framework or a separate stand-alone unit. In each case, reservoir design and implementation are very much important. The efficiency of a well-designed hydraulic circuit can be significantly reduced by the poor tank design. A hydraulic reservoir not only provides a place to put fluid. A well-designed reservoir also dissipates heat, allows air bubbles to come to the surface and dissipate, and allows time for contamination to drop out of the fluid. It may provide a positive pressure to the pump inlet and thus makes a convenient mounting place for the pump and its motor, and valves.

Hydraulic Hose

Hydraulic hose is an important part of a hydraulic system. The flexibility of hose enables components to be placed in the most efficient or convenient places, because the hose has the ability to bend around corners, through tight spaces, or across long distances.

Hydraulic hose construction

Modern hydraulic hose normally consists of at least three parts: an inner tube which carries the fluid, a reinforcement layer, and a protective outer layer.
The inner tube must have a certain flexibility and should be compatible with the type of fluid it carries. Generally, used compounds include synthetic rubber, PTFE, sometimes called Teflon, and thermoplastics. The reinforcement layer has one or more sheaths of braided wire, spiral-wound wire, or textile yarn. The outer layer is weather-, oil-, or abrasion-resistant, depending on the type of environment the hose is designed for.

Fittings and Couplings

If the components within hydraulic systems never had to be removed, connections could be brazed or welded or permanently fixed to maximize the reliability. However, it is certain that connections must be broken to allow servicing or replacing components, so removable fittings are essential for all but the most specialized hydraulic systems. To this end, fitting designs have advanced significantly over the years to improve performance and to make the installation convenient, but the overall function of these components remains nearly the same.

Fittings seal the fluid within the hydraulic system by one of two techniques: all-metal fittings depend on metal-to-metal contact, while O-ring type fittings contain pressurized fluid by compressing an elastomeric seal. In either case, tightening threads between mating halves of the fitting (or fitting and component port) forces two mating surfaces together to form a high-pressure seal.

Hydraulic seal

Hydraulic cylinder seals are used to seal the opening between various components in the hydraulic cylinder. There are mainly two types of hydraulic seals in the system:

• Dynamic seals
  Dynamic seals seal between components that are in relative motion. In a hydraulic cylinder, the rod sealing system seals dynamic reciprocating motion between the piston rod and head, while the piston sealing system seals dynamic reciprocating motion between the piston and cylinder bore.
• Static seals
  Static seals to seal between components fixed together without relative motion. Hydraulic cylinders use static seals in various places depending on the design and construction. The most common are static seals between the piston and piston rod and between the head and cylinder bore tube.

Hydraulic Fluids

Hydraulic fluids that are available nowadays serve multiple purposes. The main function of a hydraulic fluid is to provide energy transmission through the system that enables work and motion to be accomplished. Hydraulic fluids are also served for lubrication, heat transfer, and control of the contamination. When selecting a lubricant, the viscosity, seal compatibility, base stock and the additive package should be considered. Three common types of hydraulic fluids available in the market nowadays are:

• water-based,
• petroleum-based, and
• Synthetic

General Maintenance of Hydraulic System

Hydraulic attachments on construction equipment are held in position by a trapped column of oil in the cylinders and lines. If the hydraulic oil escapes, the attachment will fall.

Follow these tips before servicing construction equipment hydraulic systems.

• Before making any adjustment or repairs on the hydraulic system, it should be made sure that the ground, blocking, or cribbing is supporting the attachment, not the oil.
• The hydraulic system may have to hold pressure for a long period of time after the engine has been shut down. Removal of plugs or lines can result in oil and the component shooting out with very high explosive force. So the system pressure should always be released before making repairs or adjustments.
• Pressurized hydraulic oil leaking out of the system through a leak can be almost invisible. The oil leak should never be searched with the bare hands. A pressurized oil leak can penetrate through the skin and cause severe injury. A piece of cardboard or wood should be used when searching for suspect leaks.
• Pressurized hydraulic tanks become heated during operation; this can cause high pressure to build up in the tank. Too-quick removal of the cover can cause the hot air and oil to escape quickly. The oil may be very hot and can cause severe burns. So the pressure should be bled off before removing the cover. The manufacturer’s instructions should be consulted.
• Pressurized oil leaks spraying on a hot engine can cause disaster. Accumulated oil and fuel from leaks on the machine can even result in a fire that can spread with very explosive speed. The machine should always be kept clean and free of leaks.
• One should never try to service or repair a hydraulic system if he/she does not know about that system. This can cause serious injuries or death.
• One should never work under a hydraulic attachment which is supported on the oil system.
• One should never take chances.

The manufacturer’s instructions should always be followed for correct procedures, and one should never work under hydraulic attachments that are supported by the oil.

Steps in maintenance of the hydraulic machine

• Basic hydraulic system components are identified
• Operation of various hydraulic pumps is described
• Hydraulic system, valves, and pressure controls are serviced and troubleshot
• Knowledge of hydraulic schematic symbols is applied
• Knowledge of hydraulic circuits is applied
• Hydraulic cylinders are serviced and rebuilt

References

1. Technical book, “Construction Machinery Training”, Instate, Imlambad
2. Harris, F. and McCaffer, “Management of Construction Equipment”, Macmillan Education Ltd. London, UK.
3. Erich J. Schulz, “Diesel Equipment I and II”, Mcgraw-Hill book co.
4. Frank Harries, Ronald McCaffer, “Construction of Plant Excavating and Material Handling”, Granda Publishing.
5. SAE Handbook Volume 4
6. “Caterpillar performance Handbook”, Edition 33, Caterpillar Inc, Peoria, Illinois, USA.