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.

As shown in Figure 1, sealing devices come in two types: contact and non-contact.
Oil seals are among the major contact type sealing devices.
 

In this kind of seal, wider tolerances are possible between the OD of the seal and the seal housing. Irregularities of the housing surface can be taken up by the resilient rubber layer on the outside of the seal. However, the rubber covered seal can be blown out under high pressure in a reciprocating application whereas, with a metal case, there is no such danger.

The black color of these spark plugs is a result of carbon buildup, which is a common issue that can occur over time due to normal engine wear and tear. This carbon buildup can negatively impact the performance of the spark plug, leading to misfires, rough idling, and poor fuel efficiency. It is important to regularly inspect and replace black spark plugs to ensure optimal engine performance.