User:Automotive seals

OIL SEALS

Whenever a shaft rotates, for smooth and effective operation, lubrication is required. Wherever there’s a lubrication, seal is helping it to reach its maximum service life and reliability.

Oil seals are designed mainly to prevent oil leakage from the end of a rotating shaft, or dust intrusion from the outside air. Oil seals serve to prevent the leakage not only of lubricants, but also water, chemicals, and gas from "gaps" in machinery. Oil seals also serve to prevent the intrusion of dust, soil and sand from the outside air. Oil seals are indispensable sealing devices and are used in various fields, including automobiles, aircraft, boats and ships, railroad vehicles, construction machinery, farm machinery, petrochemical plants, electrical and home appliances.

In simplest terms

a shaft seal is a barrier. This barrier has four functions: • Retain lubricants and/or liquids • Exclude dirt/contaminants • Separate fluids • Confine pressure

When a seal performs these functions properly, it protects the bearing from harmful contaminants while retaining a clean lubricant supply, resulting in lengthened bearing service life and reliability. The bearings do their job better, cleanliness of production and process materials is maintained, lubricant is saved and machinery downtime is reduced.

Seals handle a broad range of media, from light oil to heavy grease to hot turbine gases. Electric motors and gear reduction units are some of the more common applications, however there are extremes requiring very special solutions. For example, seals protect the high speed turbine pump in a rocket that accelerates from 0 to 12,000 rpm in one-fourth of a second. Other types of seals protect the drive unit in a track type earth mover, operating at only 15 to 30 rpm.

VALVE STEM SEALS

Unlike the other seals of the engine where the goal is zero leakage, the valve stem oil seal must produce a controlled flow (regulated) leak.

Valve stem seals are devices that are engineered to allow a small amount of oil to pass through their lips (oil-metering) to lubricate the valve stem / valve guide interface of an engine

It is much more difficult to achieve controlled flow leakage because the margin for error is so small since it is so important for a thin film of oil to remain between the valve stem and valve guide. However, the amount of oil used to form this film must be strictly controlled

If too much oil passes through the lip of the seal, emissions are worsened and coke builds up on the valve, potentially causing the engine to lose power or even fail. If too little oil passes through the lip of the seal, the valve does not receive enough lubrication and will scuff, which ultimately will cause the valve to seize. The ideal metering rate lies between these two extremes.

THE MOST IMPORTANT FUNCTION Oil metering to the valve guide /stem interface is the prime function of a valve stem seal. Although the requirements for each engine differ according to the design parameters and operating conditions, the oil metering rate is generally within the range of 0.1 - 1.0mg per valve per hour.

PTFE SEALS (Polytetrafluoroethylene)

PTFE seals are assembled or machined, made-to-order radial shaft seals based on PTFE (Polytetrafluoroethylene). This thermoplastic material has  properties that exceed the capabilities of rubber compounds. PTFE seals generally require a high quality shaft finish and hardness, and are engineered to meet specific application conditions. 

PTFE offers advantages over standard shaft seal materials. PTFE seals can accept high pressures or high temperatures and high surface speeds and dry running condition and have high wear resistance and service life. Their temperature range is very wide. PTFE seals can tolerate shaft speeds of up to 12,000 feet per minute (fpm). Because of PTFE’s unique chemical structure, PTFE seals offer excellent resistance to most chemicals and fluids and they resist attack by nearly all acids, solvents and chemical agents. However, PTFE shaft seals are also with disadvantages. As with the aforementioned advantages, these disadvantages stem from the nature of the PTFE itself. Because PTFE is stiffer than traditional elastomeric lip materials, PTFE lips will not form the tiny pores (microasperities) that are integral to the pumping action seen in successful shaft seals. To compensate, a spiral groove must be machined or coined into the primary lip surface if it is intended to seal oil under low pressure conditions. This groove can simulate the pumping action by screwing fluid back into the sump, but the spiral groove limits seal usage to applications in which the shaft rotates in only one direction. PTFE also has much less “ memory” (ability to regain its original shape following deformation) than traditional, elastomeric lip materials. This reduced memory makes the lip less able to maintain consistent contact with the shaft, particularly in the presence of shaft eccentricity. As a result, leakage becomes more likely. Finally, PTFE lips are more delicate than traditional lips and can therefore be easily damaged during installation. For this reason, a PTFE seal is dispatched with pre-assembled onto a wear sleeve. This seal-sleeve combination can be slipped over the shaft with less chance of damaging the seal. Seals is also be shipped with disposable protective sleeves