Speed Reducer

Why speed reducer is needed?

• Electric motors operate most efficiently at high rotational speeds
• Typical maximum speed for an extruder motor is 2,000 rpm
• Screw speed this high would be detrimental to polymeric materials (eg. May result in excessive shear heating and polymer degradation)
  Reducer or gearbox
• High-speed drive motor is coupled to the low-speed screw using a reducer or gear box
• Typical reduction ratios, 10:1, 15:1 or 20:1
• Maximum screw speeds: 100 to 200 rpm
• Helical gears
• Worm gear for older or very small machines
• Forced lubrication system allows oil to cool the bearings and gears; this oil is water-cooled by a heat exchanger in high-load machines
  Advantage of gearbox
• Increased torque
• Due to high power consumption of polymeric materials, high torque is needed to maintain screw speed
• Most drive systems are designed to keep screw speed constant even if the torque requirement changes, which could be created, for instance, by a change in material viscosity

Types of drive systems

• Input shaft of the motor may be either directly or indirectly connected to the speed reducer
• Direct drive
  –Hard – coupled directly through gears
• Indirect drive systems
  –Utilize belts and sheaves to connect the motor to the speed reducer

Advantages & disadvantages

• Direct drive systems have better speed control and are more efficient, but may be more expensive and time consuming to repair in the case of a system breakdown
• Indirect drive systems allow more flexibility in motor location and are easier to repair if the problem simply requires a new belt

Belt as drive coupling

• For small- and medium sized extruders
• Speed – torque relationship:
  –P = NT (P=power, T=torque, N=screw speed)
• Larger gears increases the torque but reduces screw speed
• For larger drives (P>225 kW), the drive motor is coupled directly to the screw