The special mechanism used to change or restore the driving direction of a car is called the steering system. The steering system is generally composed of three main mechanisms: steering control mechanism, steering gear , and steering transmission mechanism.
The steering control mechanism is mainly composed of steering wheel , rotating shaft and steering column. The steering gear is a mechanism that converts the rotation of the steering wheel into the swing of the steering rocker arm or the linear reciprocating motion of the rack shaft, and amplifies the control force.
The steering gear is generally fixed on the frame, and the steering gear control force usually changes the transmission direction after passing through the steering gear.
The steering transmission mechanism is a mechanism that transmits the force and motion output by the steering gear to the steering knuckle connected to the wheels, and deflects the left and right wheels according to a certain relationship.
Assume that the maximum tie rod force F=15KN, the steering gear is fixed on the subframe by a combination of three identical bolts and nuts, the friction coefficient between the steering gear and the frame Ut=0.15, the ratio between the bolts and nuts Under the premise that the friction coefficient Ug=0.10~0.16, the selection process of the steering gear mounting bolts after calculation and verification is analyzed, and the calculation steps and methods of the torque method are explained.
1
Analysis of the force at the connection point
The steering gear provides steering force to the wheels through the steering rod, and the wheels react with the same force on the steering rod.
Friction and balance of this external force must be provided between the steering gear and the frame.
Friction force Fq=S╳F=1.5╳15=22.5KN, where: S is the safety factor and F is the maximum rack force.
2
Find the minimum clamping force that the bolt must provide
where: N is the number of bolts, Ut is the friction coefficient between the steering gear and the frame.
3
Estimating the bolt diameter when using the torque method d
When using the torque method for assembly, it is not allowed to over-tighten the bolts, which will cause yielding.
VDI2230 recommends that the maximum allowable assembly axial force is not allowed to exceed 90% of the yield strength (ie, the bolt guarantee load).
Considering that the axial force pre-tightening force dispersion generated during torque control tightening is about ±30%, at the same time, it is necessary to avoid over-tightening the bolts during assembly, and it is necessary to improve the use efficiency of the bolts, so when formulating the torque, the theoretical target value is usually It is to enable the bolt to provide about 70%-75% of the yield axial force, so that the maximum axial force caused by dispersion does not cause the bolt to yield, and at the same time, a certain proportion of the minimum clamping force can be guaranteed.
- Minimum clamping force of steering gear bolts: Fkmin=50KN
- Theoretical clamping force of bolts: Fm=Fkmin/70%=50/0.7=71.4KN
- Maximum clamping force of bolts: FMmax=1.3╳FKmin=1.3*71.4=92.8KN
Because , the maximum clamping force of the bolt must be less than the guaranteed load of the bolt. According to the national standard GB/T3098.1, the guaranteed load of M14╳1.5 10.9 bolts is Fs=104KN, which meets the stress requirements of the bolt.
In actual design, the determination of bolt diameter and length is often based on the BENCHMARK of competing vehicles, and reference is obtained from competing vehicles.
The length of the bolt should consider the clamping length and the thickness of the nut. Make sure that all the invalid threads at the end of the bolt are exposed, and 2-3 teeth appear to be exposed.
The total length of the bolt and the thread length should comply with the national standards as much as possible, and should be rounded to the nearest 5,0.
Based on the above requirements, Q185 M14´1.5´130 grade 10.9, hexagonal flange bolts were selected.
4
Calculate the tightening torque when using the process of tightening the bolt end
After confirming that the Q185 M14´1.5 10.9 grade bolt is the steering gear fastening bolt, dw=26.4mm is obtained.
obtained dh=15.5mm from GB5277. For this through hole size, the screw must be determined with the subframe engineer and steering gear engineer in the early stage of design, and adhere to the size specified by the national standard.
Calculate the thread pitch diameter d2=d-0.6495P=14-0.6495´1.5=13.0257mm according to GB196, and take the friction coefficient between the bolt and nut u=0.15.
Substitute the above values into
to get:
The torque in the torque table is mostly rounded to 5,0, and M=180Nm.
Calculate the torque tolerance based on the repetition accuracy of electric assembly tools ±7.5%, ΔM=180´(±7.5%)=±13.5Nm, take an integer ±15Nm.
Therefore, the design tightening torque M=180±15Nm.
5
Check the rationality of the calculated torque
1) After the bolt is tightened, when the torque is maximum, Mmax=195Nm, the friction coefficient between the bolt and the nut is minimum, when uGmin=0.10, the axial force of the bolt is maximum:
Maximum assembly axial force 95.5KN, not exceeding the guaranteed bolt load of 104KN, this check is passed.
2) After the bolt is tightened, when the torque is minimum, Mmin=165Nm, the friction coefficient between the bolt and the nut is maximum, uGmax=0.16, the axial force of the bolt is minimum:
The minimum assembly axial force is 52.8KN, which is greater than the bolt must provide Minimum clamping force, this check passed.
6
Selection of nuts
Threaded connection Design When bolts and nuts are used together, products with matching performance levels need to be selected.
Bolts with a performance level of 10.9 must be equipped with nuts with a performance level of 10. Nuts with a performance level higher than the bolts can also be used.
If the connection fails, it is always hoped that the failure mode is that the screw breaks.
The design of the nut is at least 10% of the guaranteed load of the bolt when it is over-tightened. The load failure of the bolt is the breakage of the screw. 104KN´1.1=114.4KN, the guaranteed load of M14´1.5 10.9 grade 2 type nut is 131.9KN, which meets the requirements.
The steering gear is installed on the subframe, and complex vibrations will definitely occur when the vehicle is driving.
Ordinary nuts will loosen on their own due to vibration during use. In order to prevent vibration and loosening, self-locking nuts with nylon rings are selected.
When the self-locking nut with embedded nylon ring is screwed into the bolt, the internal thread is squeezed out of the nylon ring. The extremely elastic nylon material forms a large and stable friction resistance with the bolt, thus preventing the fastener from loosening. , and at the same time, it has good absorption and damping effect on external impact and vibration, achieving reliable tightening.
The self-locking nut with embedded nylon ring is limited by the service temperature of nylon material. When using it, pay attention to the heat damage from the engine and exhaust pipe.
Based on the above requirements, Q339 M14 Grade 10 non-metallic insert hexagonal flange face locking nut was selected.
7
Calculate the tightening torque when using the nut tightening process
When tightening the nut, the friction diameter under the flange surface changes from dw=26.4mm of the bolt flange to dw=27,6mm of the nut flange. If you still need to ensure that the bolt The axial force is 71.4KN. Calculate the torque required for the nut:
. Compared with the tightening torque 184.49Nm when using the process of tightening the bolt end, there is only a 1.62Nm difference. The difference is negligible. Therefore, tighten the nut end or tighten the bolt hexagon during production. Head end, there is no difference in clamping force.
8
Consider the effect of the nut locking effective torque on the clamping force
The effective torque of the nut is the torque required to rotate the nut when there is no axial force in the threaded connection.According to GB/T 3098.9, the maximum torque for the first screw-in of the M14 grade 10.9 flange face lock nut is 31Nm. This torque will consume the input torque of the tightening tool.
According to the calculation in the previous article 5. Checking the rationality of the calculated torque, the impact of this torque consumption is reflected in the value of the minimum axial force. When the minimum torque, Mmin=165Nm, the friction coefficient is the maximum, uGmax=0.16, the minimum axial force needs to be recalculated:
At this time, the friction force provided by the three bolts of the steering gear Fq=3´43.15´0.15=19.42KN , safety factor S=19.42/15=1.3.
Under the extreme conditions of minimum torque, maximum friction coefficient and maximum nut effective torque, the safety factor of the steering gear is S1, so the selection of bolts and nuts and the development of tightening torque M=180±15Nm are safe and reliable.
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