Basic knowledge of the most fiery wheel mechanism

Basic knowledge of ratchet mechanism

basic type and working principle of ratchet mechanism

the figure shows the external meshing ratchet mechanism commonly used in machinery, which is composed of active swing rod, pawl, ratchet wheel, check pawl and frame. The driving part is sleeved on the driven shaft fixedly connected with the ratchet and connected with the rotating pair of the driving pawl. When the driving part swings clockwise, the driving pawl will be inserted into the tooth slot of the ratchet to make the ratchet turn a certain angle. At this time, the check pawl slides on the tooth back of the ratchet. When the driving part rotates counterclockwise, the check pawl prevents the ratchet from rotating counterclockwise, while the drive pawl can slide over the back of the ratchet tooth, so the ratchet is stationary at this time. Therefore, when the driving part has a compact structure: the interface is modularized, and the ratchet swings continuously, the ratchet makes a one-way intermittent movement

2 the classification of ratchet mechanism is as follows:

according to the structural form, it is divided into toothed ratchet mechanism and friction ratchet mechanism

toothed ratchet mechanism has simple structure and convenient manufacture; The time ratio between start and stop can be achieved by selecting a suitable driving mechanism. The disadvantage of this mechanism is that the dynamic range can only be adjusted in stages; Noise, impact and wear are large, so it is not suitable for high speed

toothed ratchet mechanism friction ratchet mechanism

friction ratchet mechanism uses eccentric sector wedge to replace the pawl in the toothed ratchet mechanism and toothless friction to replace the ratchet. It is characterized by stable transmission and no noise; The range can be adjusted steplessly. However, due to friction transmission, there will be slippage. Although it can play a role in safety protection, the transmission accuracy is not high. Suitable for low speed and light load

according to the meshing mode, there are external meshing ratchet mechanism and internal meshing ratchet mechanism

the pawls or wedges of the external meshing ratchet mechanism are installed outside the ratchet, while the pawls or wedges of the internal meshing ratchet mechanism are inside the ratchet. The external meshing ratchet mechanism is widely used because of its convenient processing, installation and maintenance. The internal meshing ratchet mechanism is characterized by compact structure and small overall dimension

external meshing ratchet mechanism internal meshing ratchet mechanism

according to the movement shape of the follower, after the anti sliding performance test, it can be divided into single acting ratchet mechanism, double acting ratchet mechanism and two-way ratchet mechanism

single acting ratchet mechanism. When the driving part swings in a certain direction, it can promote the ratchet to rotate. The double acting ratchet mechanism drives two pawls respectively to drive the ratchet wheel to rotate twice in the process of the active rocker swinging back and forth in two directions

double acting ratchet mechanism is often used in occasions where the load is large, the ratchet size is limited, the number of teeth is small, and the swing angle of the active swing rod is smaller than the ratchet tooth pitch

double acting ratchet mechanism

the ratchet mechanism described above can only make one-way intermittent movement in one direction. The bidirectional ratchet mechanism can realize the rotation of the ratchet in two directions by changing the swing direction of the pawl. The figure shows two forms of two-way ratchet mechanism, which must adopt symmetrical tooth shape

bidirectional ratchet mechanism

3 application of ratchet mechanism

the main uses of ratchet mechanism are: intermittent feeding, braking and surpassing, etc. the following are application examples

1. Intermittent feeding

the figure shows a shaper. In order to cut the workpiece, the planer needs to make continuous reciprocating linear motion, and the workbench moves intermittently. When the crank 1 rotates, the rocker 5 is driven by the connecting rod 2 to swing back and forth; The rocker 5 is equipped with a pawl 3 of a two-way ratchet mechanism, and the ratchet 4 is fixedly connected with the lead screw 6. The pawl drives the ratchet to rotate intermittently in one direction, so as to make the nut (experimental preparation according to the national standard gb/t 7314 ⑵ 005 "tightening method of metal materials at room temperature", i.e. workbench) make intermittent feeding movement. If the swing angle of the driving pawl is changed, the feed rate can be adjusted; Changing the position of the driving pawl (rotating 180 ° around its axis and then fixing) can change the direction of feed movement

2. Braking

the figure shows the lever controlled belt brake. The brake wheel is consolidated with the outer ratchet wheel 2, and the pawl 3 is hinged to point a on the brake wheel 4. The braking is expected to reduce the domestic waste paper import by 30% and 38% year-on-year in 2019 and 2020 respectively. The wheel is surrounded by the steel belt 6 controlled by lever 5. The brake wheel 4 rotates freely in the counterclockwise direction, and the pawl 3 slides on the back of the ratchet. If the wheel rotates in the opposite direction, the four wheels are braked

3. Override

the ratchet mechanism shown in the figure can be used to achieve rapid override movement. The movement is transmitted from the worm to the worm gear, and the output shaft 5 fixed by the ratchet wheel is driven to rotate slowly in the direction shown in the figure through the pawl 3 installed on the worm gear. When the shaft needs to rotate rapidly, the handle on the output shaft can be quickly rotated in the direction of the output shaft. At this time, because the manual speed is greater than the worm gear speed, the pawl slides over the back of the ratchet tooth, so when the worm gear continues to rotate, the output shaft can surpass the worm gear by fast manual movement

4 design points of ratchet mechanism

the design of ratchet mechanism should mainly consider: the selection of ratchet tooth shape, the determination of modulus tooth number, the determination of tooth surface inclination angle, the adjustment method of stroke and dynamic stop ratio

now take the toothed ratchet mechanism as an example to illustrate its design method

1. The selection of ratchet tooth shape

the diagram is a common tooth shape, and the asymmetric trapezoid is used for occasions with large load; When the load borne by the ratchet mechanism is small, triangle or arc tooth shape can be adopted; Rectangle and symmetrical trapezoid are used for two-way ratchet mechanism

2. The determination of modulus and number of teeth

is the same as that of gears. The relevant dimensions of ratchet teeth also use modulus m as the basic parameters for calculation, but the standard modulus of ratchet should be calculated according to the top circle diameter Da of ratchet

m = da/z

the number of ratchet teeth Z is generally selected by the service conditions and motion requirements of the ratchet mechanism. For the ratchet mechanism used for general feeding and indexing, the number of teeth of the ratchet can be determined according to the required minimum angle of the ratchet (Z ≤ 250, generally z = 8 ~ 30), and then the module can be selected

3. determination of inclination angle of tooth surface

clamped between ratchet tooth surface and radial line α It is called the inclination angle of the tooth surface. The included angle between the line O1A between the pawl axis O1 and the tooth apex A and the tooth surface normal NN passing through point a β It is called the axial position angle of the pawl

in order to make the pawl always press the tooth surface tightly and slide towards the root of the tooth in the process of pushing the ratchet wheel, it should meet the normal reaction force of the ratchet to the pawl, and the torque of n to O1 axis is greater than the torque of friction (FF along the tooth surface) to O1 axis, that is

n · o1asin β> Ff·O1Acos β

then ff/n tan

is β>

in the formula, F and are the friction coefficient and friction angle between the pawl and the ratchet tooth surface respectively. Generally, f is taken as 0.13 ~ 0.2

4. Adjustment method of stroke and dynamic stop ratio

1) adopt ratchet cover

by changing the position of ratchet cover, make part of the stroke pawl slide along the surface of ratchet cover, so as to realize the adjustment of ratchet angle

2) change the swing angle of the rocker

by adjusting the length of the crank in the crank rocker mechanism, change the swing angle of the rocker, so as to realize the adjustment of the angle of the ratchet mechanism

3) adopt a multi claw ratchet mechanism

make the angle of each rotation of the ratchet less than the center angle corresponding to a tooth γ A multi claw ratchet mechanism with m pawls can be used

for the ratchet mechanism with n=3, the positions of the three pawls are staggered in turn γ/3. When the swing rod turns Ф 1 in γ ≥ Ф 1≥ γ/When changing within the range of 3, the three pawls fall into the tooth slots in turn, and push the ratchet wheel to rotate at a corresponding angle Ф 2 is γ ≥ Ф 2≥ γ/Within 3 γ/3 integer multiples

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