1 设计目的
– clampload 夹紧力
·Design calculations are based onachieving design clamp load
·When clamp load is achieved, thejoint has structural integrity
·Over torque conditions placeincreased residual stresses in the bolt and the joint may fail when itexperiences external forces
– friction 摩擦系数防松
·There needs to be enough frictionin the joint to prevent the joint from ‘relaxing’(loosing clamp load)
2 设计原则
1. For all high strength joints the bolt should always be the weakest partduring tightening: Thus, the thread engagement length (TEL)must be at least:
MaxHardness Vickers of the bolt
TEL> 0.55 --------------------------------------------------------------d,
Min Hardness Vickers of the internalthread
where d isthe thread diameter.
2. In order to have a joint less sensitive tosettlement the clamping length divided by the screw thread diameter must always be larger than 1 (Lc/d>1);
3. The target value for the clamping force should in torque tightenedjoints be 55% of the ultimate bolt fracture load;
4. In yield point tightened joints the permanent screw elongation shall be0.14±0.12mm;
5. The resultant assembly friction coefficient must always be between 0.10 to 0.20;
6. In a dynamic high strength join the stiffness of the bolt must not bemore than 33% of the base stiffness. Or, else there is a risk for fatigue failure of the bolt if the settling is extensive;
7. Grade 12.9 screws must not be used;
8.Standard torque GB1231 shall be used;
9.Withthread forming screws the fracture torque of the joint must always be more than three times the thread forming torque;
10.Design for assembly must always be considered.
11...
3 设计考虑因数
功能，安装，几何，材料，强度等级，表面摩擦，拧紧力矩，拧紧工具
4 连接类型
动态与静态连接
1)Staticor position critical joint.Used when thejoint is exposed to an external load less than once a minute, for example safety belt attachment.Specified as “S”;
2)Dynamicor clamp load critical joint.Used when the joint is exposed to an external load more often than once aminute, for example chassis joints. Specified as “D”;
3)Semidynamic.Used when the joint is something in-between a static and a dynamic highstrength joint. Specified as “U”;
4)Threadforming joint.Joints where the bolt is thread forming, for example ground connections.Specified as “TF”.
硬连接与软连接
螺栓打紧方法
扭矩法，转角法，扭矩斜率法
5 设计步骤
1.Measure the clamping length and start with a certain screw diameterso that the joint becomes as “springlook alike” as possible
2.Estimate the external load and calculate theclamping forceneeded to secure the joint
3.Calculate what screwdimension and steel grade thatgenerate this clamping force witha standard torque.
4.After you have analyzed the scatter inassemblyfriction, set suitable assembly method.5.If torque controlled assembly is notsufficient investigate the possibility for yield point tightening.
6.Estimate the relaxation/settlement of thejoint andif needed update the design.
7.Analyze the consequence class of the joint.
6 螺栓连接与受力分析
7 扭矩，摩擦系数与夹紧力
非常重要的夹紧力来源比例： 50-40-10
影响夹紧力的因数
8 扭矩曲线
9 扭矩衰减
6inspection torque /7lower control limit –LCL
U/LSL: 图纸上规定的保证产品功能的扭矩公差范围；
装配扭矩：Target± 15%；一般与图纸规定相同；
U/LIL： 扭矩检查范围, 由实际测量情况确定；±3 sigma of measured normal
一般公差为 25%Target左右（硬连接）
LCL= LIL 0,125 x (UIL - LIL), Normally 81.5% of target / tightened Torque
10 扭矩测量
Two types of torque measurements
–Dynamic (while fastener isspinning)
·Dynamic is how much torque isapplied through the spinning gun head
–Static (after fastener hasstopped moving)
·Static is the torque required toslightly turn the joint after the joint has it has been secured with a nutrunner
measuring conditions
-Unloadedjoint
-Roomtemperature
-Timefrom assembly to inspection ≦30minutes
-Continued tightening <5º
measuring accuracy
–torque wrench ± 25%
–turn of nut ± 15%
–load washer ± 10%
–bolt elongation ± 5%
–straingauge ± 1%
11 失效模式分析
常见失效模式：
断裂：强度不足，延迟断裂，过载，疲劳，摩擦系数低
滑牙：硬度偏低，螺牙斜牙，脱碳，摩擦系数低
伸长：摩擦系数低，硬度偏低，硬度偏低
松动：扭矩不足，轴力不足，结构不合理，挤压下陷，振动
弯曲：扭矩不足，轴力不足，受冲击力
人： 未遵守操作规范
机： 拧紧系统问题，设定错误，拧紧枪套筒磨损
料： 螺纹错扣，夹杂质，批次变差大，摩擦系数变化
法： 扭矩值规范错误，拧紧系统错误，测量系统错误
环： 湿度大，温度高/低。。。
Mostcommon reason tighten NOK issues:
- Friction coefficient variation
- Batches variation-Contaminanted
- High Temperature and Humidity
利用扭矩曲线分析螺栓失效模式，如下图：
检查打紧过程的扭矩曲线记录，可以初步判断可能原因：
- No prevailing torque features
- Low friction
- Patchin the wrong position
螺栓打紧PFMEA 举例
转载自汽车智库
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