Pneumatic and Hydraulic Polyurethane Seals

One Example of the successful use of polyurethane for sealing is an Oleo-Pneumatic buffer for use on railway rolling stock. The pressures encountered in this application are not particularly high but an efficient seals is of prime importance. In this buffer compressed air or nitrogen is utilized as the spring medium for providing static resistance and recoil, the gas being sealed into the buffer behide the oil by a floating piston. It is therefore very important for the seal carried on the floating piston to be highly efficient throughout the working lift of the piston. Operational experience showed that the original seal in nitrile rubber had inadequate wear resistance and this is one of the main factor in buffer service life. After three years operational experience with polyurethane seals the outstanding properties of polyurethane were quite apparent. Seals examined from these buffers, having operated under the most arduous conditions which would have completely destroyed the nitrile seal, showed virtually no wear.

Polyurethane seals can also be used to advantage in hydraulic pit-props for roof supports and other applications. These props operate at relatively high pressure, but the seal must also prevent leakages at low pressures when the prop is out of operation.

Another use for polyurethane seals in the mining industry is found in the control valve unit of chocks. The seal in this case also acts as a seal for high pressure bleed valve. There is a high velocity flow of hydraulic fluid across the sealing face which rapidly scours nitrile rubber, whereas the polyurethane, being more wear resistant, operates satisfactorily.

Other uses for polyurethane hydraulic seals are found where the equipment is used in dirty condition, such as in the case of earthmoving equipment where the efficient operation of seals is of importance in view of hire charges and cost of down-time arising from failures.

Polyurethanes : Block and Sheet shape

Cast polyurethan elastomers, is the ease with which blocks of different sizes can be manufactured. The tooling costs are very low and due to the simplicity of shape they can be quickly made. Coupled with this is the suitability of most types of polyurethane, in particular the harder grade, for many machining operations, as already discussed in the next article. This enable in certain cases different designs to be turned out quickly fo evaluation and choice of optimum design.

Apart from blocks there is a considerable call for sheeting in various thicknesses for a wide variety of applications. Sheets are usually manufactured in a cast grade in a centrifuge. This method limits the length and width obtainable, and the largest sheets available aer of the order of 15 x 3 ft with thicknesses from 0.030 to 0.50 inches. Longer sheets can noe be made by peeling from a cylindrical block, although the thickness is limited in such cases to around 1/8 inch.

Larger sheets can also be calendered using either the millable or thermoplastic grades and the blown film extrusion of the latter materials gives very thin sheeting in long lengths. Polyurethane sheets are also available with a pressure sensitive adhesive and a non-stick removable backing paper.

Sprayable Polyurethanes

The sprayable types of polyurethane are being used where large or awkwardly shaped surfaces require covering. Before the advent of these materials castable polyurethane were used, but either the moulds required were complicated and therefore expensive. Many of the applications therefore are not entirely new but the use of the sprayable materials has resulted in lower costs and more widespread use.

One example of successful use of sprayable polyurethane is the lining of vibrator finishing barrels. The barrel are filled with an abrasive substance and untrimmed forgings and casting with the object of deburring and polishing the metal parts. The whole action is very abrasive but the sprayed coating is giving a long useful life. A slide valve on a sand hopper is another proven application, as also is the lining of a cement mixing barrel. Other uses includes the covering of wear plates in shot-blast equipment, linings of earth dumper trucks, fertilizer hoppers and even ships’s propellers.

The process is also suitable for the covering of large rollers where the tooling cost for cast polyurethanes would be prohibitive. When spraying, the rollers only need supporting between centres and rotating slowly. Similarly, the sprayable materials offer a quick and economic method for the coating of long lengths of fabric.