Easy bearing and seal replacement
Original article date: November 1999
On a Parker hydraulic tie-rod cylinder, packing and wiper seals are housed within glands which are easily withdrawn and replaced. ANDREW DELANEY explains why
One of the most suitable and successful designs for hydraulic cylinders used in industrial applications is the tie-rod type, pre-torqued on assembly to offer rigidity and stability under pressure. Once assembled, cylinders are better left undisturbed during routine maintenance, which most frequently involves the replacement of packing and wiper seals in the piston rod bearing. On a Parker hydraulic cylinder, these are housed within glands which are easily withdrawn and replaced without disturbance of the remainder of the cylinder.
High quality tie-rod cylinders have very low wear characteristics, owing to the choice of materials used in manufacture. Standard Parker Hannifin hydraulic tie-rod cylinders, for example, rated to 210 bar (typically 30% higher than normal) are fitted with hardened steel piston rods and hard chrome plated. Those fitted with non-hardened steel, even if chrome plated, are prone to denting, which leads to earlier failure of bearings and seals.
Replacement of bearings and seals on Parker hydraulic cylinders is normally straightforward and simple, requiring minimum machine downtime. A lubricated hard brass gland, which acts as the piston rod bearing as well as carrying the seals, is unscrewed and withdrawn, to be replaced with a new one pre-loaded with the appropriate seals. In this way, it is unnecessary to interfere with the end cap or ‘head’, and the pre-torqued tie-rods remain undisturbed throughout. In many instances, the cylinder does not even need to be removed from the machine.
On hydraulic cylinder designs without glands, bearing and seal replacement is considerably more complicated, necessitating the removal of the complete cylinder from the application. On this type of cylinder, once the end cap (cylinder head) is removed to replace the seals, tie-rods must be re-torqued on a flat engineering bench to ensure perfect alignment. In the case of a cylinder of 100mm bore, this would be a two-man job, as the cylinder head alone would typically weigh in excess of 5kg. By comparison, a parker removable gland for a similar sized cylinder weighs just 0.5kg and can be replaced in minutes.
Replacement glands are generally supplied with seals factory-fitted, avoiding the risk of seal damage during installation – a real danger when fitting seals directly into the end cap of a glandless design. An incorrectly fitted seal, or one which has failed to seat properly, may lead to permanent damage, further breakdown and even longer machine downtime.
Further significant advantages of tie-rod cylinders fitted with removable glands relate to inventory considerations. Replacement glands, pre-fitted with seals, are relatively inexpensive to purchase and easy to store. Compare this with the case of glandless design cylinders, where entire end caps need to be stocked, often in a number of different sizes and of differing designs to suit the mounting style of the cylinder in use.
|
Don’t forget the Pressure Equipment Directive One of the most important and far-reaching changes in industrial legislation to affect the European hydraulics industry – the Pressure Equipment Directive (97/23/EC) – became optional on 29 November 1999, but will be mandatory from 29 May 2002. It arises from the European Community’s programme for the elimination of technical barriers to trade and its purpose is to harmonise national laws of member states regarding the design, manufacture and conformity assessment of pressure equipment and assemblies of pressure equipment. Parker Hannifin is playing its part in simplifying the requirements of the Directive and identifying the choices available to manufacturers, OEMs and end users. Since the adoption of the Directive in 1997, the company’s products which fall within the scope of the Directive have been manufactured to comply fully with CEN (the European Committee for Standardisation) approval standards. The Directive requires that all pressure equipment and assemblies within scope must be safe when placed on the market and put into service. Safe pressure equipment and assemblies are defined as those which, when properly installed and maintained and used for their intended purpose, will not endanger the health and safety of persons and, where appropriate, domestic animals and property. David Piper, Parker Hannifin Cylinder Division’s Chief Engineer, New Products, is a member of the CEN TC54 Working Group 5 Standards Committee. He says: “From 29 November 1999, manufacturers could opt to apply for CEN approval for products in preference to existing European approvals such as T”V and DRIRE. Once CEN approval has been obtained, manufacturers can forget about all other national standards – at least throughout Europe. “CEN Approval is mandatory from 20 May 2002. All manufacturers will have to apply for this new approval, both for established and for new designs, before they can affix the CE mark. Manufacturers cannot avoid having to apply for the CEN approval. Existing approvals will not be accepted as equivalent.” Though manufacturers of pressure equipment and assemblies will have the choice of either following the Community regime or continuing to comply with existing national legislation, most pressure equipment and assemblies placed on the market in the UK will need to comply with the Community regime. |
- Parker Hannifin
November 1999