The mechanical design is as deep as the sea, and every small detail must be carefully considered. A slight negligence will cause troubles in the process of manufacture, use and maintenance.
For a shaft part, if the stress concentration at the sudden change in cross-sectional shape is ignored, it will increase the probability of part failure, which should be avoided as much as possible during design.
If it needs to be made into a stepped structure, a larger transition fillet can be used to reduce stress concentration. The diameters of adjacent shaft sections of the stepped shaft should not differ too much, the transition part should be gentle, the radius of the fillet should be as large as possible, and an angle transition can be used if necessary.
The seemingly similar design, the actual effect is very different. When designing non-standard parts, many unnecessary mistakes can be avoided by referring to the taboo cases of mechanical design summed up by predecessors with blood and tears experience.
01
When using flat keys to fix two parts separately, the key grooves should be on the same busbar
When using a flat key to fix two parts on a shaft, two key slots should be opened on the shaft. For the convenience of processing, the key slots should be arranged on the same busbar. If the two parts are required to be staggered at a certain angle, the keyway on the shaft should still be on the same busbar.
02
When several parts are strung on the shaft, it is not appropriate to use keys to connect them separately
If there are several parts on a shaft with the same hole diameter, it is recommended to use a connected key when connecting with the shaft. Because of the segmented connection, the directions of the keys are not completely consistent, and it is difficult to push the parts on the shaft during installation, or even impossible to install.
03
When two flat keys are used for parts on one shaft, higher machining accuracy is required
If the part and the shaft are connected by a flat key to transmit torque, when the torque is large and double keys must be used, the two keys should be located at both ends of a diameter (that is, a difference of 180°) to ensure the symmetry of the force. In order to ensure that the two keys are evenly stressed, the position and size of the key and the keyway must have high precision.
04
The distance between the two positioning pins should be as far as possible
In order to determine the position of the part, two positioning pins are often used. The position of the two positioning pins on the part should be arranged as far as possible. In this way, higher positioning accuracy can be obtained.
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05
For parts with symmetrical structures, the positioning pins should not be arranged in symmetrical positions
For parts with symmetrical structure, in order to maintain the accurate relative position with other parts, it is not allowed to reverse the installation by 180°. Therefore, the positioning pins should not be arranged in symmetrical positions to ensure that the installation will not be reversed.
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06
It must be ensured that the pins are easy to pull out
The pin must be easy to pull out from the pin hole. The methods for taking out the pin include: making the pin hole into a through hole, using a pin with a threaded tail (with internal and external threads), etc. For blind holes, in order to avoid the difficulty of assembly and disassembly caused by the air trapped in the hole, there should be ventilation holes.
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07
Compatible parts must fit easily
Interference fit parts are more difficult to load at the beginning of loading. Therefore, chamfers or guiding tapered surfaces should be designed at the entrances of the two matching parts.
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08
For shafts and hubs with interference fit, the mating surface must have a certain length
This is done to prevent shaking of the parts on the shaft. If the matching diameter is d (mm), the minimum value of the matching part length (mm) is recommended to be: Imin=4d⅔.
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09
When the interference fit and the key are used in combination, the keying groove should be installed first.
On the shaft-hub connection surface of the interference fit, the key should be inserted into the keyway first (reduce the diameter of the shaft end as shown in the left figure), and then fit into the interference fit.
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This is because when the interference fit is pressed into a section, the key and the keyway are somewhat misaligned, and the round head of the flat key cannot be used to rotate the shaft and adjust the position of the shaft to insert it into the keyway. When designing the structure, the shaft end can also be made with a larger taper to facilitate assembly.
10
Interference fit parts should have a clear positioning structure
When the interference fit is pressed in or assembled by the temperature difference method, it is difficult to control the position of the parts, and it is difficult to adjust its position after the installation is completed. Therefore, when the interference fit parts are matched, there should be positioning structures such as shaft shoulders, collars, bosses, etc., and the loaded parts are installed in place when they lean against the positioning surface.
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When it is inconvenient to make shaft shoulders, collars, and recesses, sleeves and positioning blocks can be used for positioning, and even after the installation is in place, the temporary positioning structure provided for easy installation can be removed.
11
The taper fit cannot be located with the shoulder
The mating surface of the tapered surface obtains the pressing force between the mating surfaces by axial pressing, realizes axial positioning and transmits torque by friction. For the fit of the tapered surface, the shaft shoulder cannot be used to fix the t parts on the shaft, otherwise the axial pressing force may not be obtained.
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12
The direction of the chain circlip should be adapted to the running direction of the chain
The links that connect the chain end to end are locked with a snap ring. It should be noted that the direction of the locking parts is compatible with the running direction of the chain, so as to prevent the circlip from falling off during impact, jumping and collision.
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13
Belt transmission center distance should be adjustable
The dimensional error of the belt transmission length is relatively large, and its length is constantly increasing (elongated) during work. In order to maintain a certain initial tension and realize the friction transmission between the belt and the pulley, the center distance of the pulley should be adjustable, or other tensioning devices (such as tensioning pulleys) should be used.
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14
The chain drive should be tight side up
In contrast to belt drives, chain drives should be tight side up. When the slack side is on the top, the chain and the sprocket are not easy to disengage due to the large sag of the slack side chain, and there is a tendency to get involved. This is especially the case when the chain leaves the small sprocket. If the chain is not disengaged when it should be, there is a danger of the chain being caught or snapped.
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15
In the same equipment, shafts or pins with different materials and heat treatments should have different dimensions.
In the same machine or component, the shaft or pin parts used have the same external dimensions, only the materials and heat treatment methods are different, and it is difficult to distinguish them during assembly. In order to avoid mistakes, the external dimensions of such parts should be clearly distinguished during design. the difference.
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16
The wall thickness of the lower part of the keyway of the hollow shaft should not be too thin
When using a keyed connection on the hollow shaft section, pay attention to the wall thickness of the hollow shaft. If the lower part of the keyway is too thin, it risks weakening it too much and causing shaft damage.
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17
Couplings rotating at high speeds cannot have protruding protrusions
If the heads of coupling bolts, nuts, or other projections protrude from the flange portion, the air will be agitated due to high-speed rotation, increasing loss, or becoming the source of other adverse effects. It is best if the projections are embedded in the protective edge of the coupling flange.
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18
When the transmission parts at both ends of the shaft require synchronous rotation, it is not suitable to use a disturbance coupling with elastic elements
When the two ends of the shaft are driven by a transmission member such as a wheel, it is required that the two ends rotate synchronously, otherwise there will be uncoordinated movements or jamming.
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If a coupling and an intermediate shaft are used for transmission, the coupling must use a flexible coupling without elastic elements, otherwise, due to the deformation of the elastic elements, the torsional deformation of the two ends will be different, and synchronous rotation will not be achieved.
19
When the intermediate shaft has no bearing support, do not use Oldham couplings at both ends
Because the cross disk in the Oldham coupling is floating, it is easy to cause the intermediate shaft to run unstable or even fall. In this case, another type of coupling should be used, such as a gear coupling with an intermediate shaft.
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20
The bevel gear shaft must be fixed in both directions
Regardless of the direction of rotation of the straight bevel gear, its axial force always points to the large end. However, the axial position of the shafting system should be fixed in both directions, otherwise there will be greater vibration and noise.
