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Rear Ahue of Sizaiue Berwick Car.
with SKF ball bearing research regarding fatigue,
and a set of factors deduced, as given in the
attached table. It is difficult to fix a rational method
of estimating the loads, but it was finally agreed
that for the propeller shaft and rear axle, the basis
of calculation should be maximum engine torque
in combination with a speed of 20 m. p. h., while
in the gearbox the heaviest loads operating are
calculated on maximum torque, at the
corresponding engine speed. For the hubs it was decided
to deal with the load at 20 m. p. h. and allow a
suitable factor for shock.
If the designer could fix a rational life for the
chassis in terms of mileage, the last step in the
argument would be supplied, but this
unfortunately is not possible. It is interesting to note that
tests of 20,000 miles continuous running as
conducted by the Royal Automobile Club are regarded
as creditable, and this mileage represents a factor
of unity in our bearings for the differential and
other positions for an average speed of 20 m. p. h.
Competition is naturally very keen, as the
cylindrical roller bearing, Hyatt and Timken
constructions, as well as New Departure, Double Row
Rigid, and Single Row Filling slot ball bearings,
are available. SKF propaganda for the deep
groove bearing has met with great encouragement,
as while it is agreed that the filling slot bearing
has a theoretically greater capacity, there has been
a general experience of trouble with slots. The
designer has naturally been impressed by series
of tests conducted by The National Physical La-
boratory on our behalf with our 6306 deep groove
bearing, which gave perfect performance under
varying conditions of combined overloads.
The. two extremes in automobile design are
exemplified in Figs. 1 and 2; the first represents
the Sizaire-Berwick chassis, as it existed a year or
two ago, and it is interesting to note that the factor
of safety present in the deep groove bearing E,
when fixed to deal with combined duty, is higher
than the factor present in the existing double
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