In specifying power transmission equipment, serious errors are sometimes made simply because there is not a clear understanding of the distinction between Torque Capacity and Horsepower Capacity. A capacity rating in Horsepower is often misleading because it is not always readily apparent that the Horsepower Rating for any speed reducer applies only for one specified RPM. On the other hand, the Torque Capacity Rating applies at any and all speeds for a given unit. In order to further clarify this distinction, let us consider the definition of the factors involved.
TORQUE is the product of rotative force and the radius at which it acts. An inch-pound of Torque is a unit of "twisting force" and does not involve either the speed of rotation or the distance of rotation. HORSEPOWER is a power unit representing the relation of three factors: Force acting through distance in a period of time. 1 horsepower has a Torque (Twisting Force) of 63,025 inch-pounds, when making 1 revolution (distance) in 1 minute (time). However, 1 horsepower may be any other combination of these same three factors - for instance: 1 horsepower exerts only 35 inch-pounds Torque when making 1800 Revolutions in 1 minute.
HP = (Torque in in-lbs) x (RPM) / 63,025
OUTPUT TORQUE RATING of a Universal Reducer Unit is the maximum Torque Load in inch-pounds that should be driven by that reducer unit irrespective of the RPM. This rating is on the basis of 8 to 10 hour continuous duty with a uniform load and allows a 100% Torque overload in starting.
THE OUTPUT HORSEPOWER RATING of a Universal Reducer Unit is the maximum Horsepower Load that should be driven by that reducer unit and applies for only one specified RPM. This rating is on the basis of 8 to 10 hour continuous duty with a uniform load and allows a 100% overload in starting.
HEAVY DUTY SERVICE. Service factors should be used in selecting a reducer for any load conditions except the normal 8 to 10 hour continuous duty with uniform load as listed in the Rating Tables.
SPECIFYING THE REDUCER. To specify the proper reducer, first measure or calculate the torque and horsepower requirements of the load to be driven. The reduction ratio is calculated from the required input and output RPM of the reducer. The reducer capacity required equals the actual torque load in inch-pounds times the service factor for the given conditions of load and service. All ratings are for 8-10 hour continuous duty with uniform load. For continuous duty specify the reducer model having output horsepower and output torque ratings equal or greater than the reducer capacity required. For intermittent duty, use the maximum torque capacity rating with the proper service factor. Intermittent duty is limited to drives involving 15 minutes run or less per 2-hour interval. The motorized units are furnished with the motor of your choice, but it must be noted that an 1800 RPM motor furnishes an input speed to the reducer of approximately 1740 RPM. In case of fractional horsepower motors this will be slightly less, or about 1725 RPM.
Quite often a speed reducer is bought for certain load conditions and later is used for other specifications. This drive modification is ordinarily obtained by changing the RPM of the driving motor or by using intermediate chain, belt or gear reduction in addition to the reduction in the speed reducer.
This is a very dangerous practice unless all factors are very carefully checked. To avoid serious overloading or failure of the reducer unit, check all such changes very carefully, preferably consulting the Universal Representative or our Engineering Department.
In any speed reducer installation the maximum torque output of the reducer is the vital figure to check.
NOTE - The ratings listed in the tables are calculated for an overhung load applied at a point midway between the shoulder and the end of the standard output shaft. These figures represent the maximum permissible overhung load for 8 to 10 hour continuous duty with a uniform load at a given output speed. To calculate the proper overhung or thrust load capacities for shock loads or 24-hour service, divide the above normal rating figures by service factor as determined for the given conditions from data on this page.
Total load is important to note that in the case of a combination of overhung and thrust loads, the total must not exceed the overhung load rating for that unit. This equivalent overhung load equals the overhung load plus 1.33 times the thrust load.
If the output shaft is direct coupled to the load there is no overhung load involved. If the reducer output shaft drives the load by chain, belt or gear the overhung load is the total chain or belt pull in pounds, or the force in pounds exerted at the pitch line of the gear.
Universal speed reducers are designed with a wide-bearing span and large anti-friction bearings that permit unusually large overhung and thrust loads. In addition, the ratings listed above have been calculated with a liberal factor of safety to insure long life. However, for any given installation, cutting the overhung and thrust load in half ordinarily results in increasing the bearing life ten times. For this reason it is important to specify drive equipment for a minimum amount of overhung and thrust load.
Direct coupling is the ideal design and provides for the maximum possible life for the equipment involved. In the case of any other arrangement, the longest service is obtained by using the largest possible diameter gear, pulley or sprocket on the output shaft of the reducer.
When the reducer output shaft is connected to the load by gears, chain, or belt drive rather than direct coupling, an overhung load is involved. This overhung load must not exceed the rated overhung load capacity of the unit as listed in the rating tables.
When the pitch radius is that of the gear, chain sprocket, or pulley carrying a given torque load -
Torque (in-lb) = HP x 63025 / RPM
Overhung Load = Torque Load (in-lb) / Pitch Radius (in)
If the calculated overhung load exceeds the rated capacity, a larger diameter gear, sprocket or pulley must be used. Additionally, a factor should be used depending on the type drive -
Minimum Diameter Single Sprocket = 1 x Torque (in-lb) x 2 / Overhung Load (lb)
Minimum Diameter Pinion, V-Belt, or Double Sprocket = 1-1/2 x Torque (in-lb) x 2 / Overhung Load (lb)
Minimum Diameter Flat Belt Pulley = 2 x Torque (in-lb) x 2 / Overhung Load (lb)
Reducer capacity ratings are made on a unity basis - Service factor of 1 for 8 to 10 hour continuous duty with uniform load. Service factors should be used to allow for any variation from this condition. It is evident that a larger capacity unit will be required to handle heavy duty installations involving repeated shock loads or 24-hour service, even though the actual running load be the same. The service factor is a composite figure involving the type load, the power source, and the type service. Refer to Table 1 for type load. Use Table 2 to determine the service factor for any given type load, power source, and type service. Definite recommendations can be made only after we have fully data on the actual installation.
| Table 1 - General Classifications to Determine Type Load | |||||
|---|---|---|---|---|---|
| Uniform Load | Moderate Shock Load | Heavy Shock Load | |||
| AGITATORS Liquid or Semi-Liquid CONVEYORS Uniformly loaded FANS Centrifugal or Propellor GENERATORS STOKERS |
BLOWERS Centrifugal or Vane. ELEVATORS Bucket, uniformly loaded FEEDERS Disc Type PUMPS Centrifugal Geared Rotary |
BLOWERS Lobe. MILLS Ball or Tube CONVEYORS Heavy Duty Non-Uniform Material FANS Large Mine Induced Draft HOISTS Reversing Skip |
COMPRESSORS Centrifugal or Rotatry. Reciprocating (3 or more cylinders.) ELEVATORS Freight Heavy Duty Bucket FEEDERS Pulsating Loads PUMPS Reciprocating (3 or more cylinders.) |
COMPRESSORS Reciprocating (Single or two cylinders.) CRUSHERS JORDANS PUMPS Reciprocating (Single or two cylinders.) |
CONVEYORS Reciprocating Shaker FEEDERS Reciprocating MILLS Hammer and rod PUNCH PRESSES DRAG SHOVEL |
| Table 2 - Service Factors | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Source of Power | Electric Motor Drive | Multi-Cylinder Gas or Steam Engine or Turbine | Single Cylinder Gas Engine
|||||||||
| Type Service | Inter- mittent (15 min. run in 2 hours) |
8 to 10 Hours Continu- ous Duty |
AGMA Class |
24 Hour Continu- ous Duty |
AGMA Class |
Inter- mittent (15 min. run in 2 hours) |
8 to 10 Hours Continu- ous Duty |
24 Hour Continu- ous Duty |
Inter- mittent (15 min. run in 2 hours) |
8 to 10 Hours Continu- ous Duty |
24 Hour Continu- ous Duty |
| UNIFORM LOAD MOSERATE SHOCK HEAVY SHOCK |
.8 1.0 1.5 |
1.0 1.25 1.75 |
I II III |
1.25 1.5 2.0 |
II II III |
1.0 1.25 1.75 |
1.25 1.5 2.0 |
1.5 1.75 2.25 |
1.25 1.5 2.0 |
1.5 1.75 2.25 |
1.75 2.0 2.5 |