45
5 Design Hints, Dimensions and Tolerances
5.2 Standard pulleys, flanged pulleys, idlers, clamping plates and
minimum numbers of teeth
Idlers
Idlers are either toothed or flat pulleys and do not transmit any
power within a drive system. Because of the additional flexing of
the belt, their use should be avoided where possible.
Idlers are classified according to their function as guide and/or
tensioning pulleys.
A damping pulley is recommended for high speed and/or contin-
uously shock loaded drive systems. It should be set in the slack side
of the drive just touching the belt. The relationship between the
drive pulley diameters and the drive centres in these cases should
be as follows:
2 · (d
wk
+ d
wg
) < a
The second pulley of a linear drive or conveyor system is a guide
pulley.
Within a drive system, at least one shaft should be adjustable to
permit installation and tensioning of the timing belt. If this is not
feasible for design reasons, this function must be assumed by an
adjustable tensioning pulley or idler.
Depending on the position of the idler in relation to the belt and
the other pulleys, an additional distinction is drawn between
external and internal idlers. An external idler can increase the
number of belt teeth in mesh and possibly increase the maximum
transmissible power. An idler acts in the opposite direction and can
reduce the number of teeth in mesh.
Figure 5.2: Arrangement of external idler
Figure 5.3: Arrangement of internal tensioning pulley
In order to minimise the additional bending stress in the timing belt
resulting from the use of idlers, the following guidelines should be
observed:
●
Minimum diameter of the internal pulley
≥
minimum pulley
diameter from Table 5.4. Preferably, idler
≥
diameter of the
small pulley.
●
Toothed pulleys having over approximately 41 teeth can be
regarded as flat pulleys when used as internal idlers.
●
Minimum diameter of the external idler: see Table 5.4
●
Flat pulleys should always be used as external idlers.
●
Flat pulleys must not be crowned.
●
Width of the idlers
≥
face width b
1
of the drive pulleys
●
If possible, always locate idlers in the slack side of the drive.
●
Position external idlers so that the maximum number of teeth are
in mesh on the small pulley (Figure 5.2).
●
Idlers should not be spring-loaded.
●
Keep the arc of contact on the idlers to a minimum
Standard clamp plates for Optibelt ZRL-M open-ended
belting,
for range, see page 88.
Standard aluminium clamp plates have eight teeth to accept the
end of a timing belt. The belt is then sandwiched between the clamp
plate and a machine part e.g. a tool slide (see Figure 1.4.2:
Positioning Drive, page 7).
Clamp plates are supplied without a mating back plate. The
minimum clamping lengths specified in Table 5.4 are applicable
to each end of a belt.
The clamp plate should be in the same plane as the timing belt in
order to prevent kinking in the transition between the slack side and
the clamp. If an angular deviation cannot be avoided, provision
should be made for kink free transition based on the minimum
pulley diameter involved (see Table 5.4). If there is a risk of drive
overload and there are strict safety requirements, e.g. a lift drive,
we recommend increasing the number of teeth for the end fastening
to at least twelve teeth.
Storage conditions
The cast iron and steel pulleys are protected by phosphating
against rusting with normal relative humidity in enclosed areas.
The rest of the range, including the clamp plates and bushes should
also be stored under these conditions. In the case of timing belts,
ensure storage at room temperature free from kinks.
maximum
possible distance
minimum
possible distance
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