EXTERNAL INFLUENCING FACTORS 

Several external factors affect the performance of a gearbox and need to be considered in the selection process. These external factors can be broadly classified as

• Overhung loads
• Thrust loads
• Ambient temperature and environmental conditions.

Overhung Loads

Overhung loads act on the shaft extensions when belt pulleys, chain sprockets or gears/ pinions are mounted on them to transmit power. Misalignment between two connecting shafts can also give rise to overhung loads. Overhung loads on shafts result in bending moments and additional radial loads on the supporting bearings.

The effects of overhung loads on the gearbox elements can be reduced by

• Use pulley, sprocket, pinion etc. of the largest possible diameter and locate them as close to the shaft collar as possible.

• When belt transmission is used, the belt tensions should be maintained just sufficient to ensure operation without slippage.

Overhung loads will be minimum when the power transmission is through:

• Direct coupling with capacity to adjust itself for minor shaft misalignments i.e. through Gear couplings, flexible couplings etc.
• Spline connections to shafts which are well located

The effects of Overhang loads on the gear box elements can be reduced by

• Locating the pulley, Sprocket, Pinion etc. as close to the shaft collar as possible. Special protection can be provided on request.
• Belt tension should be just sufficient to ensure operation without slippage.
• Excessive belt tension will only increase the overhand loads.
  

Allowable Overhung loads on output shafts based on L10 bearing life of 5000 Hrs. for each size are listed in Table below.

Calculation :

Allowable over hang load at any point can be calculated by the following formula

Over hang load at any point (fra):-

Fra = Frc . Frm . x / (Frm . x + (Frc – Frm) (x+y))

Where,

Frc – overhang load at shaft collar
Frm – over hang load at shaft middle
x – distance between shaft collar to shaft middle
y – distance from shaft middle
(y is +ve towards shaft end & y is –ve towards shaft collar )

Thrust Loads

Thrust loads act on the shaft of fans, agitators, extruders, screws and similar devices with axial action. freequently the working thrust is arrested on the machine itself and should not transmitted to the gearbox output shaft.

If this is not the case, the thrust load will be transmitted to the gear box causing additional axial loading of the bearings.

When the output shaft (or the input shaft) is subjected to thrust, radial or combined loading, checks for the additionally induced shaft stresses and deflection as well as for the load on the bearings should be made. Please revert to works for a correct selection of the unit.

Environmental Conditions

Standard units are designed to operate in general industrial environment. If these vary, as in applications exposed to marine, extreem cold, dusty, muddy or similar abnormal conditions, special protection may be necessary. Please refer back to us.

THERMAL RATING :

The thermal rating is the power a gear drive will transmit continuously without exceeding a maximum oil sump temperature of 90deg C. This is based on an ambient temperature of 40deg. C. For other ambient conditions, the thermal specified in the selection chart should be multiplied by the adjustment factors as listed in table given below . Thermal requirements are based on the transmitted horsepower only and account only for heat dissipated by the housing when splash lubrication is used.