Timing pulley design guidelines

by:Uliflex     2020-09-24
Other parts of the synchronous wheel (such as the ring gear, spokes, hub, etc.), in addition to strict restrictions on the quality of the synchronous wheel, usually only need to be designed according to experience, and the determined size is relatively rich in strength and rigidity. It rarely fails.
1. Tooth surface glued  
For high-speed and heavy-duty synchronous gear transmission (such as the main transmission synchronous wheel of aero engine reducer), the pressure between the tooth surfaces is large, the instantaneous temperature is high, and the lubrication effect is poor. When the instantaneous temperature is too high, the meshing two tooth surfaces The phenomenon of sticking together will occur. At this time, the two tooth surfaces are sliding relative to each other, and the bonded parts are torn, so scars are formed on the tooth surfaces along the relative sliding direction, which is called gluing. During transmission, the higher the instantaneous temperature of the tooth surface and the greater the relative sliding speed, the more likely it is to glue. For some low-speed and heavy-duty heavy-duty synchronous gear transmissions, due to the destruction of the oil film between the tooth surfaces, the adhesion failure will also occur. At this time, the instantaneous temperature of the tooth surface does not increase significantly, so it is called cold gluing. Enhancing lubrication measures, using lubricating oil with strong anti-glue ability (such as vulcanized oil), adding extreme pressure additives to the lubricating oil, etc. can prevent or reduce the adhesion of the tooth surface.

2. Gear teeth are broken   
There are many forms of gear tooth fracture. Under normal conditions, it is mainly the bending fatigue fracture of the tooth root, because when the tooth is loaded, the bending stress generated at the tooth root is the largest, plus the cross-sectional mutation and processing of the transition part of the tooth root The stress concentration caused by tool marks, etc., when the gear teeth are repeatedly loaded, fatigue cracks will occur at the tooth roots, and gradually expand, causing the gear teeth to fatigue and break. In addition, when the gear teeth are suddenly overloaded, overload breakage or shear may also occur; when the gear teeth are severely worn and the tooth thickness is excessively thinned, they will also break under normal load.
  In the transmission of helical cylindrical synchronous gears, the contact line on the working surface of the gear teeth is a diagonal line (see the illustration). After the gear teeth are loaded, if the load is concentrated, partial breakage will occur. If the manufacturing or installation is improper or the shaft is bent too much and the gear teeth are locally loaded too much, even the straight-toothed cylindrical synchronous wheel will be partially broken. In order to improve the anti-breaking ability of the synchronous wheel, the following measures can be taken: 1) Increase the root transition fillet radius and eliminate the machining tool marks to reduce the root stress concentration; 2) Increase the rigidity of the shaft and support to make The load on the gear tooth contact line is relatively uniform; 3) Use suitable heat treatment methods to make the tooth core material have sufficient toughness; 4) Use process measures such as shot peening and rolling to strengthen the tooth root surface.
  
3. Pitting on the tooth surface
  Pitting corrosion is one of the phenomena of tooth surface fatigue damage. In a well-lubricated closed synchronous gear drive, the common form of tooth surface failure is mostly pitting. The so-called pitting corrosion is the phenomenon of pitting damage caused by fatigue under the action of the changing contact stress of the tooth surface material. The initial pitting on the tooth surface is only pinpoint-sized pitting. If the working conditions are not improved, the pitting will gradually expand, and even several points will be connected together, and finally obvious tooth surface damage will be formed. During the meshing process of the synchronous wheel, the relative sliding between the tooth surfaces plays a role in forming a lubricating oil film, and the higher the relative sliding speed, the easier it is to form an oil film between the tooth surfaces and the better the lubrication. When the gear teeth mesh near the pitch line, due to the low relative sliding speed, the conditions for forming the oil film are poor, the lubrication is poor, and the friction is relatively high, especially for the transmission of a straight synchronous wheel. The force is also the largest. Therefore, pitting corrosion first appears on the tooth root surface close to the pitch line, and then expands to other parts. In a relative sense, the tooth root surface near the pitch line has the worst resistance to pitting corrosion (ie, the lowest contact fatigue strength). Increasing the hardness of the timing pulley material can enhance the anti-pitting ability of the timing pulley. Adding lubricating oil between the meshing gear teeth can reduce friction, slow down pitting, and extend the working life of the synchronous wheel. And within reasonable limits, the higher the viscosity of the lubricating oil, the better the above effect. Because when fatigue cracks appear on the tooth surface, the lubricant will penetrate the cracks, and the lower the viscosity of the oil, the easier it will penetrate the cracks. After the lubricating oil penetrates into the crack, when the gear teeth are engaged, it may be squeezed and expanded in the crack, thereby accelerating the propagation of the crack, which is a disadvantage. Therefore, it is advisable to use a higher viscosity oil to lubricate the synchronous wheel transmission with a low speed; for the synchronous wheel transmission with a higher speed (such as peripheral speed v>12m/s), use oil injection lubrication. The role of heat dissipation), at this time only low viscosity oil should be used. Open synchronous wheel transmission, because the tooth surface wears quickly, pitting rarely occurs.

4. Tooth surface wear  
  In the synchronous wheel drive, the tooth surface will have different forms of wear with different working conditions. For example, when abrasive materials (such as sand, iron filings, etc.) fall between the meshing tooth surfaces, the tooth surfaces are gradually worn and become scrapped. This type of wear is called abrasive wear. It is one of the main forms of open synchronous wheel transmission. Switching to closed synchronous wheel drive is the most effective way to avoid abrasive wear on the tooth surface.

5. Plastic deformation of tooth surface
  Plastic deformation belongs to a large category of failure modes of permanent deformation of gear teeth. It is formed by the plastic flow of tooth surface or tooth body when the tooth material is in a yielding state under excessive stress. Plastic deformation generally occurs on synchronous wheels with low hardness; but under heavy load, it will also appear on synchronous wheels with high hardness. Plastic deformation is divided into rolling plastic deformation and hammering plastic deformation. Rolling plastic deformation is formed by the plastic flow of materials caused by the mutual rolling and sliding of meshing gear teeth. Since the plastic flow direction of the material is consistent with the direction of the friction force on the tooth surface, it is crushed out of the groove on the teeth of the driving wheel along the pitch line where the relative sliding speed is zero, and on the teeth of the driven wheel The ridge is squeezed out at the nodal line. This phenomenon is called rolling plastic deformation. Hammer plastic deformation is the plastic deformation caused by excessive impact. It is characterized by shallow grooves on the tooth surface, and the orientation of the grooves is consistent with the contact line of the meshing gear teeth. To improve the hardness of the gear tooth surface, the use of high-viscosity or extreme pressure additive lubricants can help to slow down or prevent the plastic deformation of the gear teeth.
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