How to stop bolts from loosening

[Ajax Bay]

Initial signs are good that the split washers alone may do the trick for less then £1 spend! If it comes loose again within a couple of weeks I will get some loctite, but if I need to tighten it every now and then I can live with that.

Has the rack stayed tight these last 3 months?

 

[Randomnerd]

Don't we all subscribe to www.boltscience.com?
Read there a while back that washers don't help in same-metal tightening.
Looking forward to BurningLegs' feedback.

 

[Ajax Bay]

and a split washer under the bolt head

Drawing on @woodenspoons ' link: Helicalspringwashers

And more generally "Self-Loosening of Bolts and Nuts" (again thanks @woodenspoons for www.boltscience.com) precised:
Three basic locking fastener categories have been established: free spinning, friction locking, and chemical locking. (American National Standards Subcommittee B18:20 on locking fasteners)
1) The free spinning type . . .

2) Friction locking categories can be sub-divided into two groupings, metallic and non-metallic. . . . Non-metallic friction locking devices have plastic inserts which provides a thread locking function; an example being the "Nyloc" nut.
3) The chemical locking category are adhesives which fill the gaps between the male and female threads and bond them together; "Loctite" is an example . . . pre-applied to the thread.

Which is most suitable depends on the application, but in brief, the chemical locking category provides the greatest resistance to vibration loosening, followed by the free spinning locking fastener.

"In general terms, the key to preventing self loosening of fasteners is to ensure that:
1. There is sufficient clamp force present on the joint interface to prevent relative motion between the bolt head or nut and the joint. [Get the tightness/torque correct.]
2. The joint is designed to allow for the effects of embedding and stress relaxation. [Let's hope so.]
3. [Use proven thread locking devices:] thread locking compounds - such as "Loctite", flanged fasteners such as "Whizlok" or torque prevailing fasteners such as "Nyloc". In general, loose washers, of the plain or spring variety, are not generally advisable."