General information
about electric chain hoists
Apart from the usual criterion such as lifting capacity, lifting speed and dimensions also consider following:
1. Choosing a motor according to FEM 9.682
In addition to the torque the decisive criterion for rating an electric motor is the heat it generates. Here we differentiate between two operational modes.
1.1 Intermittent duty
In this case the motor is designed for a series of equal
cycles consisting of duty periods with constant load and rest periods. The heat
generation depends on the relative duty cycle, that is, the relationship
between operating period under load, total operating time and the number of
starts/hour.
Duty
rating ED = (Operating period / Operating period + rest periods) %
The
number of cycles that can be made under full load is calculated as follows:
S ≈
0,3 x ED x V / H
·
S = Cycles per hour
·
ED = Duty rating in %
·
V = Lifting speed in m/min.
·
H = Average lifting height in m
A
cycle consists of a motion of lifting, lowering and the rest periods. One must
ensure that the lifting height does not exceed the value permitted by the
percentage duty cycle referred to a cycle period of 10 minutes.
H ≤
ED x V / 20
and
that simultaneously the permissible number of starts is not exceeded. It is
generally accepted that a cycle consists of 6 starts.
1.2 Short time duty
Where
special duty conditions exist (e.g. long hook path) the operating period must
be of such length that the admissible temperature limit of the motor is not
exceeded. For such cases intermittent duty must be replaced by short time duty.
That is, the motor may be operated for up to 10 starts over a certain period
(usually 15 min.). Thereafter the motor must cool down to room temperature.
2. Classification of
hoisting equipment according to FEM 9.511
To
choose an optimal hoist the lifting capacity and also the classification group
must be known. The classification group indicates the theoretical operating
time of the mechanical components under full load:
Classification group |
1Bm |
1Am |
2m |
Operating time in h |
400 |
800 |
1600 |
If
the hoist is operated as classified an actual operating time of around 10 years
can be expected. After this period a general overhaul is necessary. To define
the classification group following values must be determined:
2.1 Average operating
time per day
The
average operating time can be estimated or calculated as follows:
Operating
time/day = (2 x average hook path x cycles /hour x operating time/day / 60 x
lifting speed
Electric
hoists provided with counters for operating hours and number of starts
simplifiy the classification
2.2 Load spectrum
The
load spectrum indicates to what extent a hoist or part thereof is subject to
maximal stress or whether it is subject to smaller loads only. It can be
calculated or estimated according to the diagrams on the right:
2.3 Classification
The
classification group is defined by operating hours and load spectrum: