Zinc diecastings can either be threaded or holes can be produced to take through bolts. In most cases, for ambient or moderately elevated temperature applications, it is most cost effective to thread the zinc diecasting or to use thread cutting or thread rolling bolts or screws. This is because reasonably long holes with sufficient accuracy to be directly threaded can be cast in zinc. The mating part then simply needs a short through hole, which is readily produced by other processes in other materials if required (eg aluminium diecasting or steel pressing).
There is usually no need to depart from standard thread sizes and forms that perform well in the majority of situations. Thread strengths in zinc are excellent, with normal amounts of thread engagement it will be found that the bolt head will break off before the thread in the casting strips. There is not much difference in strength between cut and rolled threads but if anything the advantage lies with the latter, however cut threads reduce the amount of subsequent torque relaxation. Thread rolling using fluteless taps has the significant advantage of producing no swarf. It should be remembered that the hole size required for roll threading is larger than that for a cut thread. The table below gives recommended sizes for drilled parallel holes and cast tapered holes for standard metric threads.
Recommended Hole Sizes Prior to Threading
|For Cut Threads
|For Rolled Threads
and Taptite Screws
For components that will be subject to elevated temperatures in service the stresses that are induced by bolt clamping loads need to be considered. These stresses will undoubtedly relax over time and hence the clamping loads will be reduced. This effect is commonly called torque relaxation because one of the ways it manifests itself is a reduction in the torque required to unscrew the bolt. For detailed information on this subject see “Joining Zinc Diecastings with Threaded Fasteners”
The load required to pull steel bolts out of threaded holes in ZP3 has been determined over a range of bolt diameters, engaged thread lengths and for several types of thread. All the results can be rationalised to give the single band shown in the chart below
The relationship (over the test range) is:
Pull-out load in Newtons =90∏DL
D=bolt diameter, mm
L=engaged thread length, mm
No significant difference was found between tapped threads or self-tapping screws.
Load at which a bolt will strip female alloy ZP3 zinc threads, in a tensile test.
Data obtained on diameters of 3 to 6mm with thread lengths of 1-6mm.
The standard equation relating applied torque T(Nmm) to the bolt clamping load F(Newtons) and the bolt diameter D(mm) is F=T/kD
The coefficient k can vary widely from the usually assumed value of 0.2. Measured values for zinc-plated screws in zinc alloy are 0.2 to 0.3 at diameters below 5mm and 0.15 at 8mm diameter.
Note that with time the preload will relax. If this is an important factor see “Joining Zinc Diecastings with Threaded Fasteners”
In the chart below the torque at which a zinc plated steel bolt will strip the threads out of a ZP3 casting is plotted against a rationalised parameter. Self-tapping screws require higher torque as some of the force goes into cutting the thread. Over the limited test range the relationships are:
Tapped:Tmax = 0.08D2L
Self Tapping: Tmax = 0.12 D2L
T= Torque (Nm)
D= Bolt diameter (mm)
L= Engaged thread length (mm)
The results imply a high frictional coefficient between the zinc plate and the zinc alloy. It is possible that a lubricated bare steel screw would cause thread stripping at half the indicated torque.
Torque required to strip female zinc alloy ZP3 threads
Data for diameters of 3 to 5mm with thread lengths of 1 to 6mm. Bolts were zinc plated (k =0.3)
(Note: lubricated bare steel might fail at half indicated value)
Care should be taken in extrapolating the data beyond the test envelope, especially the torque information. The safe loads that may be continuously applied will be lower than the instantaneous strength values given here. Further advice should be sought.
Male Threads in Zinc Alloy
The data above is for steel bolts in zinc “nuts”. The information should also provide guidance on the behavior of male zinc threads but care should be taken. Note that shear or tensile failure of the bolt core is also possible failure modes.
Thread Strengths in Zinc Alloys Other than ZP3
No data is available on other alloys. It is probable that thread strengths will be proportional to alloy shear strengths. Hence the values given for ZP3 in the charts above could be multiplied by the following factors.
ZP5 times 1.2
ZP2 times 1.5
ZP8 times 1.3