No force should be used when mounting the impeller When mounting the impeller in agricultural and low
power applications, the fitting between hub and shaft is often
unnecessary tight. Trying to mount the impeller in such a
situation often results in forcing the hub onto the shaft. It is
important, however, not to use any force when mounting the impeller
because force can result in burrs at the keyway as the hub slides onto
the shaft.
The solution
The solution is not so simple. On the one hand it is important not to
use force in order to prevent burrs at the keyway. On the other hand it
is important to use a tight bore because the appearance of even a
diminutive rotational slip will build up and lead to severe stress on
the key and keyway. A close fit between hub and shaft is significant
for the overall quality of the impeller.
As mentioned in the teaser the most commonly used bore tolerance on
standard bosses is H7, which provides a close fitting between hub and
shaft. The H7 is defined by being a positive tolerance where the size
of the bore is bigger than, or equivalent to, the size of the shaft.
The F7 tolerance is also a positive tolerance, however, where H7 has a
tolerance of +18µ/-0µ for a 14mm hub, F7 has a tolerance of
+34µ/+16µ. Thus F7 allows a bigger clearance between hub
and shaft than the H7 tolerance does.
When it comes to agricultural and low power applications, the solution
is to use a F7 tolerance instead of H7, because the F7 tolerance makes
it possible to have a relatively tight bore and mount the impeller
without force. As long as the hub is kept perfectly straight, it will
slide right on the shaft using no force.
We have recently invested in new equipment so that it is possible for
us to manufacture a F7 bore tolerance. We are now able to
manufacture a 19mm hub using a F7 bore tolerance.
