SubscribeIn order to meet the diverse requirements of the most varied applications, manufactures offer a broad range of speed reducers product lines featuring different configurations. For their part, design engineers must make the most of such a versatile offering so to optimise that power transmission section of their applications.
Speed reducers are used in a broad range of machines with the most varied torque and speed requirements , and the number of possible combinations becomes virtually countless when one considers the many different reducers configurations available on the market.
The reason for this diversified offer is that the demands on speed reducers are not limited to speed reduction or torque multiplication, but frequently include load bearing functions or supporting the mass of the applications. Because of this structural function reducers are called to perform in the machine they are incorporated into, the design and relative position of reducers shafts play a major role in reducer selection.
Speed reducers are normally classified by the relative position of input and output shafts. This is the concept behind such terms as ‘in line’, ‘parallel-shaft’ or ‘right angle’ (i.e. helical bevel) gear units, as opposed to worm gear units, traditionally identified by the type of gearing rather than by shaft arrangement.
For each specific application, a design engineer is likely to identify two or more families of reducers that meet calculated torque and speed requirements. As a result, a design engineer will need to evaluate different factors in order to establish which, among compatible configurations, best suits that specific application.
Besides a variety of technical considerations, the limiting factor frequently turns out to be space availability. As a matter of fact, the growing demand for compact machines is placing increasing emphasis on the space efficiency of motion control systems.
Understandably enough, design engineers frequently have to compromise on performance where space costraints prevent using the technically ideal solution.
Where no limiting factors ovverule technical considerations, design engineers are free to make the most of the features and advantages offered by each product line. A brief outline of said features is provided:
- Worm reducers: Their typically low efficiency makes them the ideal choice for low or middle -to-low power applications featuring intermittent duty. Worm reducers are also recommended for non-reversing applications, however for gear ratios i>70-80 only. Worm reducers are better suited to withstand overloading generated by loads fluctuating widely overtime or shock loads than other designs. Among other advantages, the worm design allows for very high gear ratios, up to i=100 per single stage, which translate into greater cost effectiveness. Finally, worm reducers typically ensure quiet,vibration free operation.
- In-line reducers: This style is normally characterised by high torque density, ie transmitted torque per unit of volume. These reducers represent the natural complement to electric motors. Driven machine can be controlled directly by the parallel output shaft or via an external transmission (belt, chain or gear). Two bearing sites for the output shaft ensure good overhung load capability, normally sufficient for applications with the above mentioned requirements. Available in a wide range of ratios, these reducers cover a speed range in the order of 3< of terms in efficiency space enhanced for motor compact whit versions as well configurations, mount flange and foot such option, various offer manufactures Major requirements. speed lower will motors 6-pole with combinations while 4-pole combined when rpm 500 < meet>
- Right-angle reducers: In this configuration, input and output shafts are right angled thanks to a bevel gear set with intersecting or non intersecting axes. The right-angle design ensures great space efficiency in terms of width and provides the only alternative to worm reducers in applications involving such space limitations.
The right-angle design, furthermore, is the preffered choice where efficiency is the primary consideration, for instance in continuous duty applications, requiring high motive force or using expensive prime movers, such as brushless motors, brake motors, switch pole motors, mechanical or electronic variators.
These reducers normally come in a wide range of versions. Most interesting is the version with hollow output shaft, with or whitout shrink disc. In this configuration ,the gearmotor is fitted directly onto the parallel shaft of the driven machine, resultingin enhanced space efficiency in terms of width. In this case, a torque arm must be installed, normally available in the list of options offered by any manufacturer.
- Shaft-mounted gear units: These reducers feature a flat, elongated housing. Typical of this family is the hollow output shaft, featuring a shrink disc or a keyway. The whole purpose of the shaft mouth style lies in its inherent space efficiency, which makes it the ideal selection for many applications thanks to such features as: simple, neat installation; ease of installation/removal with no need to remove any machine parts; costly machining of mating surfaces and shaft alignment are omitted; shrink disc version ensures low backlash, since keyway and its additional backlash are omitted; torque arm can be combined with torque limiting devices, such as load cells. In the configuration featuring a solid input shaft, typically driven via an external belt transmission, final speed can be adjusted - within limits - by simply changing either of the pulleys so to modify the primary transmission ratio.
Finally, space efficiency is further enhanced by carefully selecting installation, for instance opting for the ‘U’ rather than ‘Z’ configuration, so to reduce axial dimensions to a minimum. Because of their particular configuration, and the advantages listed above, shaft mount reducers are typically used in conveyor belts in a number of different industrial applications.
- Parallel shaft reducers: In all manufactures’ ranges, these reducers are recommended for severe heavy-duty applications, with installed power ranging from a few kW to 200 kW and above. These reducers use sturdy bearings, frequently the straight or taper roller type, suitable to withstand the high radial and thrust loading typically encountered in such applications as: mixer, stirrers; wood, stone, ore crushers; extruders for plastic materials; bucket elevators, conveyor belts; dies and winding machines; ventilators and compressors; roller ways, etc.
Selecting the frame size
Once the product configuration/type that best suits the application has been identified, a design engineer will inevitably be faced with the selection of the appropriate size of gear. Let us assume that our design engineer has completed all static and dynamic calculations concerning the application, and the parameters listed below are know (the abbreviations used here are Bonfiglioli Riduttori’s standard symbols):
Mr2 -Torque required at gearmotor output shaft; n2- Gearmotor output speed; fs- Service factor for the application; R2 - Radial loading (if any) onto output shaft.
The load charts included in all technical manuals may help determine the severity of applications duty cycle, ie the ‘service factor - fs -’. The diagram supplied by Bonfiglioli Riduttori may also prove useful. The diagrams uses these inputs:
z no. of starts per hour
h/d daily operating hours
K1/K2/K3 Curves of load inertia where K = Jl
Jl Moment of inertia of the load (referred to motor shaft)
Jm Moment of inertia of the motor
When output speed n2, torque demand of the application Mr2 and estimated efficiency
h are know, power requirement at reducer input shaft Pr1 can be calculated as follows:
Pr1 [kW] = Mr2 [Nm] x n2 [min-1]
h
After calculating the input power value, the normalised electric motor with the next higher rating is selected, assuming a continuous duty S1. Avoid selecting an electric motor with exceedingly high rating compared to calculated power, as this - besides increasing the cost of installation - may lead to abnormal conditions upon starting or in operation and place exceeding stress on reducer and other components of the drive train.
Consider also that efficiency h and power factor cos j are adversely affected when load in operation is lower than rated load, meaning that a motor is more efficient when operating close to nominal conditions.
Once motor power Pn, operating speed n2 and obviously the type of reducer are know, the specific unit that meet the application requirements can be selected from the rating charts included in the catalogue. Safety factor S must be equal to or greater than the service factor of the application, ie S > fs. Once selection is complete, it may be appropriate to double check on few parameters. Some of the conditions to be verified are listed below:
Radial load applied to input and/or output shaft must be lower than the permitted value specified on the catalogue: R=Rn.
In this regard, remember that belt transmissions will put more stress on shafts.
Mechanical power applied to reducer must be lower than the thermal rating for that reducer in actual operating conditions: P1= Pt. Typically, thermal factors involve significant constraints for large size reducers featuring low gear ratios.