hi-lok sizing

huangyhg

hi-lok sizing
i have a task where i have to pick grip lengths for hi-lok fasteners.  what is the best way to calculate the correct length?  we have a requirement that two threads be showing past the nut.  what confuses me is all the possible tolerance stackups of the material and fastener and the spec tells you what the max and min grip length can be calculated.  i don't have must experience with this and i know is a standard calculation that has been done a zillion times in the past.
thanks
mark
the standard procedure for this is to assume nominal thicknesses for each of your component parts and allow a substitution of up to 2 grip lengths to accomodate tolerances.
individual practices vary at different companies, but that method is pretty standard.
watch out for the "two thread" rule with hi-loks.
there is a specific go/no-go dimension for hi-lok pin protrusion measured at the collar.
textron sells special hi-lok go/no-go gauges, but gives the dimension in the fastener installation manual.
the specific data for the fasteners can be had at their web site.
the easiest way is to use a hilok grip gage.  the gage has lines and numbers that identify the correct length needed for installation.  usually what ever number is showing subtract one and that should be the correct length.  even though you may have a requirement for two threads you have to ensure that you don’t shank out the nut on short thread length hiloks.  i find that as long as the crown of the hilok is protruding past the nut it is acceptable.  what your really looking for is that the locking feature on the nut is completely engaged on the threads.
i have also seen that the last two threads on a hi-lok are not rated or roll thread controlled or tested.  we actually had to ground a few airplanes because of this.  the fasteners were not protruding from the nut elements in a wing spar.
as superstress stated every company has their own specification for this.  allow for nominal and some part tolerance.  washers can always be added to the nut stack up to allow for varying grip lengths.  just be careful not to let the nut shank out.  it is very difficult for the mechanic to remove a fastener of this type if there is no hex drive recess in the tail of the fastener.  this may result in the fastener spinning during fastener removal and will mechanically and thermally damage the hole.  
but obviously don't make the grip length too short so that you have threads in bearing.  we usually kept the grip length out of the drawing requirements and mandated the mechanics actually grip the fastener during assembly or repair to allow for any tolerance build ups for the actual fastener installation.