Transmission Gear Drive
Ian Gibson • Spur Gear
An elementary mechanical System
• Distance – Most common
• Type of motion – Capable of large gear reduction
• Change in load – Noise and Stress during collision impact
• Speed Tooth geometry - Involute profile
• Applied force Features
• Direction Contact point move
Shape compensation
M-1 Result: Constant ratio of rotational speed
1-2 2-L
Gear Drive
• Helical gear
– Solution to chatter problems with the Spur
gear
Prime Component Component Load – Curved teeth enables the load to be spread
mover No.1 No.2 more evenly
– Can be used for change in direction (90o)
Figure 1.1 Elementary mechanical system Gear Drive
•
Source: Mechanical System Components, James F. Thorpe,1989,Allyn and Bacon
Bevel gears
Shafts
– Changing orientation
• Usually round, either solid or tubular
– Designed to work at a range of angles
• Strength and rigidity is the main design
– Straight or curved teeth
consideration Gear Drive
Coupling
• Basic function: transmit torque
• Worm gears
– Large gear reduction (spur)
• Accommodate misalignment and end
– Worm can turn gear but not vice versa
movement, shock absorption
– Phenomenon is due to friction and angle of
• Most are better than 99% efficient
forces
• Loss is through friction heat Gear Drive
Clutches • Rack and pinion
• Couple, decouple to allow – Converting rotation to linear motion (more
acceleration/deceleration of mechanical components on this later)
and maintain at appropriate velocities – Not for continuous motion
Harmonic Drive Gearing
Chain Drive
--- Basic Components
• Flexible as belts Wave Generator
• Positive as gears •
•
Elliptical in shape
Built-in ball bearing on outer
• Excellent design flexibility circumference of elliptical cam
• Resistance to shock loading • Usually used as input element
• Efficiency at 97 to 99% Flexspline
Belt Drive • Elliptical in shape
• Clean (no lubrication required) • Flexible, thin-walled, external teeth
• Two less teeth than circular spline
• Transmit over wide selection of speed ratios • Rotate with the wave generator
• Can be used as fixed or output member •
Circular Spline
• Thick-walled, internal teeth, rigid steel
ring
• Can be used as output or fixed member
Harmonic Drive Gearing
--- Principle
Harmonic Drive Gearing
--- Principle
Speed reduction ratio:
input velocity no. of teeth of
flexspline
C-Beam Drive of
output velocity no. of teeth of flexspline – no.
X-ray Machine
of teeth of circular spline
Rotary to Linear Motion
where circular spline is fixed and flexspline is used
as output • 3 common approaches
– Timing belt
For example: – Pinion and rack
• 202 teeth of fixed circular spline – Ball and lead screw
• 200 teeth of output flexspline Rotary to linear motion:
Belt Drives
Speed reduction ratio = 200/(200-202)=-100
negative sign indicates input and output are in • Cleanliness
opposite directions. • Low cost
Harmonic Drive Gearing • Lubrication-free
--- Characteristics • Absorbs shock loads
• High single-stage reduction ratio • Can provide variable speeds
- 30:1 to 320:1, dual drives can make ratios from • Quiet operation
1,000:1 to 100,000:1 • High efficiency
• Zero backlash • Transmits power between widely spaced
• High precision shafts
- positional accuracy around 30 arc/sec and
• Wide selection of speed ratios
repeatability within a few seconds
• Small-sized and lightweight for high ratio
Belts
applications
Flat belt
• High torque capacity
• Rubber with
- torque is transmitted through multiple tooth contact
synthetic fibre reinforcement
• High efficiency
• High friction levels
- 85%-90% with no decrease in efficiency with
• High tension is not required
increase in ratio
• Misalignment
• Vibration-free operation
Applications
- teeth do not come into rolling contact with one
Pinion and Rack
another so operation is quiet
• Consists of two parts
• Back (reversible) driving
– Rack: flat and toothed part
• Minimal wear and long life
– Pinion: gear part
- tooth friction losses and wear are negligible due to
• Diameter of the gear determines the speed
essentially zero sliding
that the rack moves as the pinion turns
- flexible steel flexspline has a higher resistance to
Backlash
fatigue
• Central hollow shaft
• Causes problems when the drive direction is
Application:
reversed, there is a short delay before the driving
C-Beam
gear teeth mesh with the driven teeth
Ball and Lead Screw – Gear teeth cut into the outside of it, which
• Used in many precision motion systems engage a gear that moves the pitman arm
– Steering wheel connects to a threaded rod
• There are two types of screw system for
that sticks into the hole in the block
rotary to linear motion – When the steering wheel turns, it turns the
– Lead screw bolt.
– Ball screw
• Can move heavy loads with ease thanks to
the mechanical advantage of the threads
• Nature of thread count and pitch restricts
these linear motion devices to moving load quickly
Back Drive
• Vertical actuators can suffer from back drive
• Resisted by system forces
– Mechanical inefficiencies
– Static friction in the bearings and the drive
system
– Reduction (timing belt or gear) inefficiency
• Pitch of a screw-driven system also
contributes to resist back drive
– 2-rev/mm ball screw has a lower back drive
than a 5-rev/mm screw
• If power fails, system inefficiencies resist
the load's inclination to back drive.
• If the load is heavier than the back drive
forces, it will fall uncontrollably.
• Method used to prevent back drive
– Place a fail safe brake on the motor or ball
screw
– Degrade the efficiency of the system
Lead Screw
• Advantage:
– Moving heavy loads at a fairly low price
– Back-drive does not occur
Not possible to push the nut down the screw,
safe for vertical axis positioning
– Quiet
– High precision
Ball Screw
• Advantage:
– Friction can be reduced due to the ball
(lower torque will be required to move the load)
– Higher speed capabilities
– Higher precision
– High duty cycles as less heat builds up in
ball screw drive systems when compared to
conventional screw drives
Recirculating Ball Screw
• Recirculating-ball steering gear
– Consists of two parts
– Block of metal with a threaded hole in it
