Fishplate bolts

[Astro]

Can any one tell me why the bolts holding flishplates have a square head and not the standard Hex head? Thanks for your help.

 

[Cherry_Picker]

To make it more difficult for pikeys to nick them?

 

[hwl]

From a highly qualified engineer not railwayman so don't take this as being the whole story.

I have seen both square (older) and hex (new installation and replacements)

Square nuts have less risk of the corners rounding as they regularly get tightened over the decades.
If access to tighten nuts isn't much of an issue there is less need for nuts to be hex.
Historically (victorian era) many more nuts were originally square than the present hexagonal, the (old) square nuts in general would have been made from plate (cheaper) whereas hex are completely forged (more expensive but less so now).
Square have a larger flat area than hex to spread the load.

The new ones with hex flats also appear to be all metal locking type nuts so they don't vibrate loose gradually (--> less frequent inspections).

I would expect the answer to be victorian standard that kept going as there was no need to change for very long time.

 

[Trog]

Most of the older fishbolts are pan pear, with a round head. Rotation of the bolt while the nut is done up, being prevented by a pear shaped section on the bolt stem under the head that locks in the similar shaped hole in the first fishplate.

 

[Joseph_Locke]

Most of the older fishbolts are pan pear, with a round head. Rotation of the bolt while the nut is done up, being prevented by a pear shaped section on the bolt stem under the head that locks in the similar shaped hole in the first fishplate.

Square heads on flat bottom assemblies are retained to make their engagement with blocks and fishplates more positive and stop them rotating whilst torquing up the other end.