Several things. Heat and cold cycles and age eventually harden the rubber compound being used. It also slowly fatigues the spring behind the lip. Older leather lip seals dried up if they were allowed to sit for long periods, plus they simply wore out. If you ever look at a National Seal size catalog, there are many pages describing lip styles and various rubber compounds to use depending on the application, rotation speed, and the type of fluid to be sealed. Back when they first started using rubber seals in automotive applications, the material, Buna-N, I believe, worked well, but it hardened up after a few years. The newer rubber compounds they use today include polyacrylate and others that have a WAY longer life, and new ideas about lip designs have extended the efficiency of seals greatly. Contaminants in the fluid are also a factor in determining how long a seal will last.
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Today's Featured Article - New Life for an Old Allis - by Tyler Woods. My friend Jon, has an old '39 Allis Chalmers B. He thought it a marginal tractor that had long since served its time. She smoked terribly and never had much power but he couldn't afford another so he was limping along with what he had. Jon's Allis has a small front loader and though it doesn't carry much, it serves his needs. It was the hard starting and low power that made him think it was time to replace the old girl. Jon called me to help him discover why his tractor wouldn't start
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