When transmitting load, the gear of the gear reducer box will produce thermal deformation, which is due to the friction heat generated by high-speed rolling and sliding between the tooth surfaces. In addition, the high-speed rotation of the gear, friction blast and bearing friction also produce heat. One of these heat is taken away by the cooling oil circulation and radiated outward through the oil-gas space. After heat balance, the remaining heat will remain in the gear body. Make the gear temperature rise and deform. For high-speed and wide helical gear reducer, due to high temperature and uneven distribution along the gear, uneven thermal expansion causes helix deviation. Therefore, even if the tooth surface contact is uniform during assembly, the distribution of load along the tooth width will still be uneven during operation.

According to some experiments of gear temperature field, for spur gear reducer, it is usually higher in the center of tooth width, while the temperature at both ends of the tooth is relatively lower due to better heat dissipation conditions. The highest temperature part of helical gear reducer is offset. This phenomenon is caused by the axial flow of lubricating oil from one end of meshing to the other end, and the highest temperature at the tooth width of about 1 / 6 from the side of meshing end is caused by hot oil.

Other factors that affect the load distribution include the error of gear helix angle, the deformation of gear box and frame, the axial offset of bearing clearance caused by the direction of load and the radial displacement caused by the centrifugal force of high-speed rotation of gear body.