CAPÍTULO V De la Inversión Pública
Artículo 62. La Secretaría de Agricultura, de acuerdo con lo que establece la Ley de
In order to dimension the variable speed gear units correctly, the installation altitude, ambient temperature and operating mode must be known in addition to the required power and speed setting range. The output power Pa, efficiency η and slip s are
illustrated as a function of the gear ratio i in the following diagram.
Dimensioning criteria
Since mechanical variable speed gear units are not only speed variators but also torque variators, they must be dimensioned according to various criteria:
– According to constant torque – According to constant power
– According to constant torque and constant power (each for a partial speed range)
This figure displays the characteristics of Pa, s and η corresponding to the measurements of loaded variable speed gear units. The diagram shows a close connection between efficiency and slip and the specified gear ratio. For mechanical reasons, such as maximum friction between belts (friction disc) and maximum circumferential velocity as well as speed-dependent friction factors, there are no linear relations in this case. In order to ensure optimum use of a variable speed gear unit, a differentiated examination of the type of application is required.
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Figure 18: Variable speed gear unit parameters
Pa= power
η = efficiency s = slip
i0 = gear ratio of the variable speed gear unit
0 1 2 3 0 0 0 3 0.3 i0 S [%] 0.9 Pa Pa s Gear ratio
na0= output speed without load ne0= input speed without load
Dimensioning for constant torque
Most drive applications require substantially constant output torque over the setting range. Variable speed gear units designed for this purpose can be subjected to a torque that can be calculated with the following formula:
In this arrangement or operating mode, the coupled reduction gear unit is uniformly loaded over the whole setting range. The variable speed gear unit is fully utilized only at the maximum speed. At lower speeds, the required power is less than the permitted power. The smallest output at the lowest speed in the setting range is calculated using the following equation:
The following figure illustrates torque and power as a function of the speed: Output torque
Ma = output torque [Nm]
Poutmax= maximum output power [kW] noutmax= maximum output speed [min–1]
Output power
Pamin = minimum output power [kW] R = speed setting range
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Figure 19: Parameters of the variable speed gear units at constant torque
Pa max(n) = maximum power according to test
Definition torque Ma= limiting torque Ma maxof the reduction gear unit
0 0
0 0
na min na max na na min na max na M Ma M = const.a Pa Pa min Pa max Pa max(n)
1
2
3
4
5
6
7
8
9
10
11
12
13
Dimensioning for constant power
The output power Pacan be utilized throughout the entire setting range and can be calculated using the following formula:
The variable speed gear unit is fully utilized only at the lowest output speed. The coupled reduction gear unit must be able to transmit the resultant torques. These torques may be some 200 – 600 % higher than in the constant torque arrangement (see characteristics).
Output power
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Figure 20: Parameters of the variable speed gear units at constant power
Pa max(n) = maximum power according to test
Definition torque Ma= limiting torque Ma maxof the reduction gear unit
0 0
0 0
na min na max na na min na max na
Pa P = Pa a min= const. Ma Ma min Ma max Pa max(n) Ma max Ma
Dimensioning for constant power and constant torque
With this load type, the utilization of the variable speed gear unit is at its optimum. The reduction gear unit is to be dimensioned so that the maximum output torques can be transmitted. The power remains constant in the range na’ ... namax; the torque remains constant in the range namin... na’ .
If the available speed range of the variable speed gear unit does not have to be fully utilized, it is expedient for reasons of efficiency to use a higher speed range. In the upper speed range, the slip of the variable speed gear unit is in fact at its lowest value and the transmittable power at its greatest.
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Figure 21: Parameters of the variable speed gear units at constant torque and constant power
Pa max(n) = maximum power according to test
Definition torque Ma= limiting torque Ma maxof the reduction gear unit M (t) = permitted torque characteristics
Output power
1
2
3
4
5
6
7
8
9
10
11
12
13
Service factors The following service factors are important when selecting the correct variable speed gear unit on the basis of the selection tables:
• fB= service factor for load type (see the following table)
• fT= service factor for the effect of the ambient temperature (see the following figure) The total service factor is the product of fBx fT.
Overload protection
The existing motor protection, regardless of its type, does not protect its following gear units.
Electronic overload protection system
Electronic monitoring may be employed to protect the following gear unit stages of variable speed gear units against overloading. The motor output and output speed of the variable speed gear unit are measured by the electronic overload protection system. At constant torque, the power alters in linear fashion with the speed, i.e. the motor power drops with decreasing speed. If this is not the case, the drive is overloaded and then switched off. The overload protection system is not suitable as protection against stalling.
Overload-limiting couplings are suitable components for protection against stalling.
Load type fB Notes Examples
I 1.0 uniform, smooth operation fans, light conveyor drives, filling machinery
II 1.25 non-uniform operation with moderate impacts and vibration
cargo lifts, balancing machinery, crane drive units
III 1.5 highly non-uniform operation with violent impacts and vibration
heavy mixers, roller conveyors, punching machinery, stone-breaking equipment
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Information on project planning
The dimensioning of variable speed gear units depends on various parameters. The following table contains the most important information on project planning for VARIBLOC®and VARIMOT®.
Criteria VARIBLOC®(belt gear unit) VARIMOT®(friction disc)
Power range 0.25 ... 45 kW 0.25 ... 11 kW Setting range 3:1, 4:1, 5:1, 6:1, 7:1, 8:1 depending on the
pole number of the driving motor and the input power.
4:1, 5:1 depending on the pole number of the driving motor and the input power.
Adjustment at standstill
Adjustment at standstill is not permitted as the tension of the belts is adjusted automatically during operation only.
Adjustment at standstill is permitted, but it should not be carried out regularly. Load type Suitable also for alternating loading (impacts
due to supply of material, etc.), damping through the belt.
Suitable for uniform loading only (e.g. conveyor belts), the friction disc can slip in case of load impacts and the surface may consequently be damaged.
EX protection For the definition of explosion protection for mechanical variable speed gear units, see "Drive Engineering – Practical Implementation – Explosion-Protected Drives." All
transmission belts are electroconductive and prevent static charging through rotating parts. Actual value encoders with evaluation and disconnect are employed to monitor the minimum speed if the speed falls below the minimum speed. Use inverter drives for operation in hazardous areas.
See "Drive Engineering – Practical
Implementation – Explosion-Protected Drives" for the definition of explosion protection for mechanical variable speed gear units. The friction ring is electroconductive and prevents static charging through rotating parts. Actual value encoders with evaluation and
disconnect are employed to monitor the minimum speed if the speed falls below the minimum speed. Use inverter drives for operation in potentially explosive atmospheres.
Wear The belt is a wearing part, which must be replaced after being operated under rated load for approx. 6,000 h. The service life increases considerably with less load on the unit.
Low rate of wear, detailed instructions for replacement intervals are not available.
Adjustment possibilities
Handwheel or chain sprocket, electric or hydraulic remote adjustment.
Handwheel, electric remote adjustment. Indicator units Analog or digital indicator units, analog display
with special scale.
Analog or digital indicator units, analog display with special scale, setting indicator displayed on the housing.