Washers - what are they for? Do lock washers really lock? Do lock nuts? + other locking methods
Washers help distribute load and prevent the screw head from digging into the joint material. If the surface of the joint isn't smooth, it's more likely that the screw will compress higher spots over time and come loose. Also, if the surface is damaged by the screw or nut, it can lead to problems with future re-installation. It's important to use a washer that's hard enough for the given screw. For instance, be sure to use a hardened washer for high-strength screws and bolts (Grade 8 and socket head cap screws). Locking Devices: There are several different types that ostensibly help to keep a joint from loosening. Their effectiveness depends on the applicat ion and is somewhat debateable. * features that supposedly prevent loosening: a spring action and an edge that digs into the screw on reversal. According to this thread <http://www.eng-tips.com/faqs.cfm?fid=1257> it seems that everyone from NASA to British defense to the US navy thinks split lock washers are useless. Some of the rationale include the fact that the spring force of the washer is only around 5% of the force from the stretched bolt, and that the edge cannot dig into anything when it flattens out. Yet, millions of these washers are used every year, so one would think they're not completely useless. According to the "Handbook of Bolts and Bolted Joints," by Bickford, p. 243, the lock washer undergoes additional deformation after it flattens with a spring rate more comparable to that of the bolt. This extra springyness is helpful for preventing fatigue failure, but it's unlikely that it helps prevent loosening. These washers are probably most effective in joints where the recommended tightness cannot be achieved, such as soft metal, plastic or wood joints. In these cases, the washer would likely not be entirely flat and would indeed dig into the screw surfaces. * Toothed washers: These have small teeth that dig into adjacent screw and joint material. The consensus seems to be that these are more effective than split lock washers, but will (and must) cause damage to adjacent surfaces, which may affect repeated installation. * Belleville washers: These are cone shaped (not shown) washers are used more as a precision spring than a locking device. They can be stacked to increase their combined spring rate (see the wiki <http://en.wikipedia.org/wiki/Belleville_washer> ). Their spring rates are substantially higher than split lock washers. They may provide some protention in high vibration or temperature changes. Wavy washers have a similar purpose. * Sems: These are screws with freely spinning captured washers--washers that are permanently attached. See some pictures here <http://www.instockfasteners.com/PRODUCTS/sems.asp> . * Fender washers: These have a much wider outer diameter than typical washers and are useful on softer materials. * Loctite: This is actually the prefered method for securing screws against vibration. Loctite is like a glue that hardens when oxygen is removed. The most common type can be removed by heating the joint. * Castle Nuts: These have slots that accept a cotter pin that goes through a drilled hole in the bolt. * * Lock Wires: Bolts heads with holes are strung together with wire so that they cannot turn relative to each other. Used for vibration resistance and as an anti-tampering device. * Lock Nuts: The most common type is a nut with a nylon insert. These are very effective (certainly more so than lock-washers), but may not work for reassembly. There are also nuts called "prevailing-torque" locknuts. These have warped threads or tapered features that apply increased friction on the bolt. [Non-text portions of this message have been removed]
