QUESTION
I have just purchased a light weight steel flywheel from Lotus Marques and would like to fit a starter ring-gear to it
How hot do I have to heat the ring-gear so it expands sufficiently so it will fit on?
M.B. - Melbourne, Australia
ANSWER
One Australian starter ring gear manufacturer ACL, recommend their ring-gear should be heated to between 240 degrees to 260 degrees Centigrade.
However they make no reference to critical dimensions and how these temperatures are determined.
Understanding the practice of a shrink fit ring-gear is about school-boy physics and the coefficient of linear expansion.
So to understand the issues that are involved in this exercise, some assumptions have to be made -
1) The outside design step diameter of the flywheel to locate the ring-gear is assumed to be 10.050"
2) The inside design diameter of the starter ring-gear to locate on the flywheel is assumed to be 10.025"
For the purpose of calculation, the above measurements provide an interference fit of 0.025"
In the real-world, this interference fit maybe quite different because of manufacturing tolerances.
In addition to the this, the teeth of the ring-gear will be flame hardened and this will result in a distorted ring-gear.
Heating the ring-gear in an oven will cause the component to expand to a point where it can be fitted to the flywheel.
Once the ring-gear is removed from the oven it will immediately begin cooling.
Therefore it is necessary to over-heat the ring gear to compensate for the small delay involved during the fitting process.
I have just purchased a light weight steel flywheel from Lotus Marques and would like to fit a starter ring-gear to it
How hot do I have to heat the ring-gear so it expands sufficiently so it will fit on?
M.B. - Melbourne, Australia
ANSWER
One Australian starter ring gear manufacturer ACL, recommend their ring-gear should be heated to between 240 degrees to 260 degrees Centigrade.
However they make no reference to critical dimensions and how these temperatures are determined.
Understanding the practice of a shrink fit ring-gear is about school-boy physics and the coefficient of linear expansion.
So to understand the issues that are involved in this exercise, some assumptions have to be made -
1) The outside design step diameter of the flywheel to locate the ring-gear is assumed to be 10.050"
2) The inside design diameter of the starter ring-gear to locate on the flywheel is assumed to be 10.025"
For the purpose of calculation, the above measurements provide an interference fit of 0.025"
In the real-world, this interference fit maybe quite different because of manufacturing tolerances.
In addition to the this, the teeth of the ring-gear will be flame hardened and this will result in a distorted ring-gear.
Heating the ring-gear in an oven will cause the component to expand to a point where it can be fitted to the flywheel.
Once the ring-gear is removed from the oven it will immediately begin cooling.
Therefore it is necessary to over-heat the ring gear to compensate for the small delay involved during the fitting process.
Heating temperature of shrink-fits
Shrink-fits are assembled by heating to temperatures where the expansion exceeds the interference. The necessary temperature change can be calculated as -
dt = δ / α di (1)
where
dt = temperature difference (oC, oF)
δ = diametric interference (mm, in)
α = coefficient of linear expansion (m/moK, in/inoF)
di = initial diameter of hole before expansion (mm, in)
Diametric interference can be calculated as
δ = dt α di (2)
Substituting dimensions, temperature change and the coefficient of linear expansion for steel results in the following calculation -
Temperature change = 240 - 20
(240 degrees is assumed to be oven temperature)
(20 degrees is assumed to be ambient temperature)
(coefficient of linear expansion for steel is 0.000016)
(initial diameter of hole ring-gear before expansion is 10.025")
δ = (240-20) x 0.000016 x 10.025 = 0.035"
So from this calculation it would be safe to assume the ring-gear is going to expand sufficiently to fit on the flywheel.
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