Common failure mode cause analysis
There are many scenarios where bolt connection fatigue failure occurs, including toro zero turn parts and toro zero turn mower parts, from aerospace to bicycles.
①Assembling, twisting, pulling：
Torsional fracture is characterized by obvious necking and elongation in the fracture zone. The common causes of torsional fracture of parts such as toro zero turn parts and toro zero turn mower parts are mainly due to the low friction coefficient of the connection surface; the torque applied during tightening or pre-tightening is too large, and the torque applied by the sleeve and the thread is different; the shaft speed and the applied torque are too fast; the performance strength of the part itself is not enough, and the perpendicularity between the tightening surface and the center line of the thread is out of tolerance.
②The thread is twisted off by shear force:
Fractures twisted by shear forces are generally spiral-shaped, with no significant necking. The common reason why the thread is twisted by shear force is that the thread is stuck during the tightening process, such as: thread deformation, connecting teeth, etc. The shape is inconsistent, and there is welding slag on the thread; The part that the bolt is screwed in is clogged, and if the nut is a blind hole, the effective thread depth is not enough.
③Breaking after use at stress-concentrated areas:
The fracture after use at the stress concentration part is usually manifested in the head of the bolt and the right angle part where the head and the threaded rod are excessive. The common reason for the fracture of the stress concentration part is that the fillet of the right angle part between the head and the threaded rod is too small; the bolt is cold-formed There are defects in the plastic streamlines of the head. The verticality between the connected surface and the bolt is out of tolerance.
④Fatigue fracture:
The main fracture of bolted connections of parts such as toro zero turn parts and toro zero turn mower parts during use is fatigue fracture. Common causes of fatigue fracture are: insufficient pre-tightening force; excessive attenuation of clamping force; unqualified bolt size and performance; the interaction between parts and the assembly environment and operating conditions do not meet the design requirements.
⑤Delayed rupture:
A common cause of delayed fracture is hydrogen embrittlement. Hydrogen embrittlement is a trace amount of hydrogen that enters the interior of the steel during the production process (such as electroplating and welding), causing the material to embrittle or even crack under the action of internal residual or external stress. Common fasteners prone to hydrogen embrittlement include: self-tapping nails/elastic washers/bolts with electroplated surface treatment above grade 8.
⑥Part torque alarm:
Part torque alarms often occur during the bolt assembly process where torque is controlled by the angle method. The failure modes and causes of fastener torque alarm include: after the assembly is completed, the final torque of the part is higher than the upper control limit or lower than the lower control limit: the reason is that the assembly torque control range of the part is unreasonable, which is manifested as excessive set control range. Small, the control range shifts upward or downward.
There is no pre-tightening to the preset angle, and the torque reaches the upper limit and the alarm occurs: the reason is that the friction coefficient of the part itself exceeds the upper limit, the friction coefficient of the parts fits exceeds the upper limit, and the interference between the parts causes the assembly torque to rise sharply.
Normal assembly, torque lower limit alarm: The reason is that the friction coefficient of the part itself exceeds the lower limit or the friction coefficient of the parts fits exceeds the lower limit. When the parts are screwed in, the fitting torque is greater than the initial torque (that is, the screwing torque consumption is too large), which is common in locking nuts. Tighten.
⑦Thread sliding thread:
Thread slippage often occurs in threaded connections. The main cause of thread slippage is thread decarburization: a common phenomenon is that the torque cannot be added during assembly. After the bolt is removed, it is found that all or part of the thread is smoothed, and the bolt thread or nut The surface hardness of the hole is low; the dimensions of the internal and external threads are matched: the contact area of the matching coupling pairs is small. There are two situations: one is that the number of thread threads for tooth engagement is small, and the other is that the threads do not contact within the pitch diameter (that is, the precision fit is not Well, there is not enough contact between the bolt threads and the nut threads).
At the same time, if the assembly method is not the right hole, strong tightening will also cause thread slippage; The coefficient of friction of the thread is too small: the surface coating, surface roughness, surface lubricant is unreasonable, and there are foreign objects in the bolt thread or threaded hole, which damages the thread. Changes in pitch and angle of bolts and nuts can also cause thread slippage. Disclaimer: The above content is from [SOLIDWORKS non-standard mechanical design] and is only for communication and learning. If you have any questions or objections, please leave a message to contact us