Coefficients of Friction for Various Horological Materials
Posted by Ed Hahn on January 31, 2000 at 13:09:13:
TZ Classics Forum Number: 1091
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We all know that synthetic sapphire is used in various parts of the watch as a load-bearing surface. These include wheel pivot jewels (i.e. the bearings), pallet jewels which lock the escapement and receive impulses when unlocked, and the roller jewel which the pallet fork "kicks" to keep the balance wheel moving.
As has been documented extensively, synthetic sapphire is used because it is a very hard (Moh's scale 9) yet slick surface. However, not all pivot bearings are made of sapphire, especially in vintage watches of less than the highest grade. Sometimes brass bushings are used in place of sapphire, while in other watches steel bushings are used.
Here is some friction data on some of the more common surfaces which come into contact in a typical watch. (Source: CRC Handbook)
(Note - these values are for static friction - which is essentially what must be overcome at every pivot every time the escapement is locked and unlocked. Dynamic friction, or sliding friction associated with movement, is generally less than these values. The lower the number the less friction.)
sapphire-on-sapphire - clean or lubricated: 0.2
sapphire-on-steel - clean or lubricated: 0.15
steel-on-steel - no lubricant: 0.58
steel-on-steel - animal oil lubricant: 0.085 to 0.10
steel-on-steel - mineral oil lubricant: 0.13 to 0.18
steel-on-brass - no lubricant: 0.35
steel-on-brass - mineral oil lubricant: 0.19
steel-on-brass - castor oil lubricant: 0.11
steel-on-brass - fatty acid lubricant: 0.13
From these figures, one might conclude that:
1) steel-on-steel with animal oils (e.g. traditional watch oils derived from fish) gives the lowest friction of all of the above surfaces, but steel-on-steel gives a pretty high friction once the lubrication's effectiveness is lost. Thus, while it is vitally important for pin-lever and bushed mechanisms to be lubricated regularly before the lubrication dries out, once in this period of high wear, the watch should definitely show signs of reduced power reserve.
2) steel-on-sapphire gives a reasonably low friction, regardless of lubrication state. However, I'm assuming that wear on the steel pivots is higher when unlubricated. Therefore, it is even more important for jeweled lever / pivot movements to be regularly cleaned and lubricated - as the watch may continue to work fine even if the lubrication is dried out.
I wonder how well these conclusions are borne out in the field...