Locking principle of pressure washer
The principle behind the anti-loosening function of a lock washer is that once compressed, it generates a constant elastic force that keeps the threaded connection between nut and bolt under tension and creates a resisting torque, thus preventing the nut comes loose.
Additionally, the sharp edges of the lock washer in the opening are pressed into the surface of the screw and the connected component, inhibiting any relative rotation between the screw and the connected part.
How about the anti-loosening effect of the pressure washer?
Spring washers are often used in structural and non-structural components of various mechanical products due to their low cost and ease of installation. They are particularly suitable for parts that require frequent assembly and disassembly.
However, the anti-loosening ability of pressure washers is limited. In developed countries such as Europe and America, its use is decreasing, especially in crucial load-bearing connections, due to the demand for high reliability.
In some military industries, the use of pressure washers has been replaced by stainless steel. It is reported that the use of steel pressure washers has been banned by the Chinese Academy of Space Technology for safety reasons, including the risk of hydrogen embrittlement and fatigue.
Pressure Washer Lock Box
The connection between the shaft final drive and the shaft housing uses class 10.9 M16 x 100 screws with a tightening torque of (280+20) N·m, tightened with a high-precision electric screwdriver. The change in torque with rotation angle during the screw tightening process was measured with and without spring washer.
Comparison of the torque-angle curves showed that there was always a pre-tightening torque of approximately 10 N·m when a lock washer was used, while without the lock washer the screw torque remained at 0 N·m before increasing significantly. This suggests that the lock washer can be completely flattened with a screw pre-tightening torque of about 10 N·m. Confirmation of this inference was obtained through inspection with a digital torque meter, which showed that the screw torque did not reach 20 N·m and the pressure washer was completely flattened.
These results indicate that the lock washer only provides an elastic force of 10 N m, which is insignificant compared to the screw pre-tightening torque of 280 N m. Furthermore, such a small force is not enough to embed the sharp corners into the lock washer groove on the screw surface and the connected part. After disassembly, no obvious marks were observed on the surfaces of the screw and the connected part.
As a result, the anti-loosening effect of the lock washer on the bolt may be disregarded.
Adding a washer between the screw and the connected part can present another potential safety hazard if the washer has quality issues. When the screw torque is high (greater than 200 N·m), using a spring washer as a locking mechanism is not effective and may even be harmful. Under the influence of impact, vibration and variable loads, the preload may suddenly disappear, causing the connection to loosen.
NASA also recognized the limitations of open pressure washers. In the fastening nut chapter of the NASA standard, it states that “The typical helical lock washer…serves as a spring while the bolt is being tightened. However, the washer is typically flat when the bolt is fully tightened, at which point it is equivalent to a solid flat washer, and its locking ability is non-existent. In short, this type of spring washer is useless for locking.”
See the figure below for excerpts from the original text. However, some professionals have differing opinions. What is your perspective on this?
