For the past 2-3 weeks I have been researching the coarse spline shaft that is in my tractor. I found out that Massey Ferguson tractors built before 1971 all have core spline shafts in them. A spline is basically in layman terms a gear cut onto a shaft. Coarse refers to the size and type of splines (like teeth on a gear) on the shaft.
Usually coarse spline shafts have up to 9 or 10 splines and are gennerally on small shafts. The splines are usually big and bulky and can have an effect on the performance output as well as the size of force the shaft can withstand. The material strength is a similar issue to bolts: Mild steel bolts are like putty, all the ones in our cars are high tensile of varying grades, some stronger than others depending on the application.
The ultimate strength of a bolt is reflected in what torque it should be tightened to so if we look at 3/8 UNF bolts in MGA/B/Midget as an example, the more normal high tensile bolts torque to about 30 lb-ft, head studs 45-50 lb-ft, and crown wheel bolts to 60 lb ft. Halfshaft and gear box component steels have a similar range. Shown below is what a coarse spline shaft looks like:
The other part of the strength equation is the design. The performance of a halfshaft of a given material is influenced by two primary considerations. The root or core diameter (that’s the diameter of the base of the splines) and the concentration of stress. If a given amount of stress is concentrated into a smaller area then the shaft is more heavily loaded and more likely to fail.
The standard halfshafts of the early Massey Ferguson tractors like the 20, 35, 65 are the worst possible design:
The large square, coarse spline form reduces core diameter and also focuses stress into the centre of the halfshaft, while having the spline cut into it, it causes further stress raiser at the end of the spline. Waisting the halfshaft down to the root diameter is a good way of optimising the strength of what comes standard. It’s not so much that removing metal makes it stronger, it’s that it spreads the stress along a greater length of halfshaft thus reducing the concentration of stress substantially.
Fine spline is based on the same concept except it holds a greater number of splines (like teeth on a gear) and is generally on a bigger shaft.
Other design features which reduce stress concentration substantially are finer splines, v-shaped splines, and also the splines being formed by rolling, as per the later Massey Ferguson halfshafts.
A further advantage of the fine spline form is that the core diameter is increased about 20%, giving directly a similar increase in strength. Shown below is what a fine spline shaft looks like:
A common misconception is that harder halfshafts are needed. While a certain degree of hardness is required of course to withstand the load applied to the spline without it wearing away, hardness isn’t the issue, it is the core strength of the material that’s important. The stronger steels also tend to be harder, but aren’t hardened as such, they can still be cut and machined etc.