Cycloidal drive
Encyclopedia
A cycloidal drive or cycloid
al speed reducer is a mechanism for reducing the speed of an input shaft
by a certain ratio
. Cycloidal speed reducers are capable of high ratios in compact sizes.
The input shaft drives an eccentric
bearing that in turn drives the cycloidal disc in an eccentric, cycloidal motion. The perimeter of this disc is geared to a stationary ring gear and has a series of output shaft pins or rollers placed through the face of the disc. These output shaft pins directly drive the output shaft as the cycloidal disc rotates. The radial motion of the disc is not translated to the output shaft.
, causing the cycloidal disc to move in a circle. The cycloidal disc will independently rotate around the bearing as it is pushed against the ring gear. This is similar to planetary gears, and the direction of rotation is opposite to that of the input shaft.
The number of pins on the ring gear is larger than the number of pins on the cycloidal disc. This causes the cycloidal disc to rotate around the bearing faster than the input shaft is moving it around, giving an overall rotation in the direction opposing the rotation of the input shaft.
The cycloidal disc has holes that are slightly larger than the output roller pins that go inside them. The output pins will move around in the holes to achieve steady rotation of the output shaft from the wobbling movement of the cycloidal disc.
The reduction rate
of the cycloidal drive is obtained from the following formula, where P means the number of the ring gear pins and L is the number of pins on the cycloidal disc.
Single stage efficiency approaches 93% and double stage approaches 86% with single stage reduction ratio up to 119:1 and double stage up to 7569:1
Also, the eccentrically mounted cycloidal disk will cause vibration in the drive which will propagate through the drive/driven shafts. This will also cause increased wear on the exterior teeth of the cycloidal disk, as well as the interface with the output roller pins due to small relative motion caused by the vibrations. A second cycloidal disc installed a half-rotation relative to the first will balance the input shaft and reduce vibration.
Cycloid
A cycloid is the curve traced by a point on the rim of a circular wheel as the wheel rolls along a straight line.It is an example of a roulette, a curve generated by a curve rolling on another curve....
al speed reducer is a mechanism for reducing the speed of an input shaft
Driveshaft
A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...
by a certain ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
. Cycloidal speed reducers are capable of high ratios in compact sizes.
The input shaft drives an eccentric
Eccentric (mechanism)
In mechanical engineering, an eccentric is a circular disk solidly fixed to a rotating axle with its centre offset from that of the axle ....
bearing that in turn drives the cycloidal disc in an eccentric, cycloidal motion. The perimeter of this disc is geared to a stationary ring gear and has a series of output shaft pins or rollers placed through the face of the disc. These output shaft pins directly drive the output shaft as the cycloidal disc rotates. The radial motion of the disc is not translated to the output shaft.
Theory of operation
The input shaft is mounted eccentrically to the ball bearingBall bearing
A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit...
, causing the cycloidal disc to move in a circle. The cycloidal disc will independently rotate around the bearing as it is pushed against the ring gear. This is similar to planetary gears, and the direction of rotation is opposite to that of the input shaft.
The number of pins on the ring gear is larger than the number of pins on the cycloidal disc. This causes the cycloidal disc to rotate around the bearing faster than the input shaft is moving it around, giving an overall rotation in the direction opposing the rotation of the input shaft.
The cycloidal disc has holes that are slightly larger than the output roller pins that go inside them. The output pins will move around in the holes to achieve steady rotation of the output shaft from the wobbling movement of the cycloidal disc.
The reduction rate
Gear ratio
The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...
of the cycloidal drive is obtained from the following formula, where P means the number of the ring gear pins and L is the number of pins on the cycloidal disc.
Single stage efficiency approaches 93% and double stage approaches 86% with single stage reduction ratio up to 119:1 and double stage up to 7569:1
Disadvantages
Unlike many other drive mechanisms, the input and output shafts of the cycloidal drive can not be reversed. This is because rotating the output pins will not rotate the cycloidal disc in the correct way.Also, the eccentrically mounted cycloidal disk will cause vibration in the drive which will propagate through the drive/driven shafts. This will also cause increased wear on the exterior teeth of the cycloidal disk, as well as the interface with the output roller pins due to small relative motion caused by the vibrations. A second cycloidal disc installed a half-rotation relative to the first will balance the input shaft and reduce vibration.
See also
- Epicyclic gearingEpicyclic gearingEpicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear...
- Cycloid gearCycloid gearThe cycloidal gear profile is a form of toothed gear used in mechanical clocks. The gear tooth profile is based on the epicycloid and hypocycloid curves, which are the curves generated by a circle rolling around the outside and inside of another circle, respectively.When two toothed gears mesh, an...
– A gear tooth shape based on the cycloid - Harmonic driveHarmonic DriveA Harmonic Drive is a special type of mechanical gear system that can improve certain characteristics compared to traditional gearing systems . It was invented in 1957 and is now produced by Harmonic Drive LLC...