Oil seal basics: how o-rings, seals are designed.

Tip: oil seal Basics

Most of you will already understand these basics of how an oil seal is designed but we wanted to put this tip out for people new to the field.

In an oil seal environment there is a shaft that is rotating inside a housing or bore. Because of friction, there must be some clearance between the shaft and the bore. The function of the oil seal is to stop whatever fluid is inside from leaking out the clearance between the shaft and housing. The seal may also be used to prevent outside materials, such as dirt, from moving in through the clearance.

"Dynamic sealing" is the relationship between the rotating shaft and the seal and is handled by the sealing element. A garter spring may used in the oil seal to increase the radial interference between the seal lip and contact point on the shaft. In order to achieve this interference, the oil seal's ID must be slightly smaller than the diameter of the shaft.

"Static sealing" is the relationship between the housing and the seal. In order to achieve this interference, the oil seal's OD must be slightly larger than the diameter of the housing or bore.

An oil seal normally consists of three basic components: the sealing element, the metal case and the spring. The purpose of the sealing element is to stop the fluid from leaking between the shaft and housing. The metal case will give rigidity and strength to the seal while it is being held in the bore or recessed groove. The spring will help make the sealing element more effective. All materials must be selected depending on the environment in which the oil seal will function.

What are Oil Seals - Guide to what an Oil Seal is used for

What are Oil Seals – A guide to Oil and Rotary Shaft Seals

Also known as a Rotary Shaft Seal, Shaft Seal, Lip Seal, Elastomeric Lip Seal or any variation of these. It is a simple device for excluding dust, dirt, water or any other contaminant whilst retaining lubricant in rotary shaft equipment. Generally, it has been developed as a means of protecting the bearings of rotating shafts.

In this blog, we attempt to highlight what are Oil Seals and the various Rotary Shaft Seals including Mechanical Face Seals, Water Pump Seals, Gland Packings, and V-Seals that are readily available.

What are Oil Seals and how do they work?

The basic principle of sealing is straightforward – the flexible lip is held against the rotating part (usually the shaft) whilst the casing (or O.D.) is pressed into the housing or bore and holds the seal in place. The sealing lip needs some form of lubrication to avoid overheating and is usually energized by means of a garter spring.

What are Oil Seals and the different types?

Many – too numerous to list, covering a vast range of designs, sizes, and materials suitable for a never-ending range of applications. Some designs conform to International Standards such as BS1399 and DIN 3760 for metric sizes and seal types, but the majority have been manufactured to suit particular applications – hence the enormous selection available. This blog is intended to assist in this selection and will consider seal type, materials, and sizes.

What materials are available?

Leather is probably the oldest of the lip materials still in common use, but the move towards mass production methods has seen a massive increase in the development of synthetic rubbers which lend themselves to accurate and repeatable injection and compression moulding. Nitrile (NBR) is still by far the most common elastomer for “normal” use, whilst Viton® (FKM/FPM) is rapidly replacing Polyacrylate (ACM) and Silicone (VMQ) for high-temperature applications. Viton® also has high resistance to abrasion and chemical attack making it a preferred elastomer. Recent developments in the use of PTFE for Rotary shaft seals has caused widespread interest particularly for high-speed shaft rotation or poor lubrication applications.

How are they used?

Once you have selected the most suitable seal available, considering the environment, temperature, shaft speed, pressure, lubrication availability, as well as the size, of course, the seal should be stored adequately and then fitted properly. Here are a few suggestions that could help:-

Storage and Handling

There is a British Standard laid down for the control of synthetic rubbers. BS 3574 (1989) helps to determine shelf life – for instance, Nitrile (NBR) and Polyacrylic (ACM) are Group ‘B’ rubbers and have a 7-year life, whilst Silicone (VMQ) and Fluoroelastomers (Viton®) are Group ‘C’ rubbers and have a 10-year shelf life. PTFE and Leather do not come into this category but like the others should be kept in the original packing for as long as possible away from direct light, dust, and humidity. Ozone, which can also be produced by battery-driven forklift trucks has a very bad effect on synthetic rubbers. Finally, protect the sealing lip – DO NOT hang the seals on nails, wire etc.

Installation

If the seal is being fitted to original equipment you may have some influence over the shaft and housing bore finish, but if you are replacing a worn seal you still need to take into account the condition of these 2 essential parts. Check for sharp edges and burrs – particularly on the shaft and housing chamfers or you could ruin the seal before you start up. If the shaft is too worn consider using an M Barnwell Services Shaft Repair Kit.