Spline Drive Fasteners 3

[Fabrico]

Hello all,
Although I'm an aviation enthusiast, I stay on the ground and build race cars. Occasionally I get my hands on and use beautiful spline drive fasteners. Information on these is scarce. Some seem to be among the highest strength aerospace fasteners out there. I have seen them on things like military landing gear, but also on jet engines. Is the spline just to make them idiot proof in war time or? I am somewhat familiar with Mil specs and fastener numbers but can anyone shed some light on their general use and why the spline drive.
Thank you!

 

[TVP]

Spline and 12-point (double hex) drive features were created because the extremely high strength materials used for aerospace fasteners cause tool problems during installation with conventional hexagonal drive features. Remember that many aerospace bolts and screws are heat-treated to tensile strengths > 200 ksi (1400 MPa) and therefore are used at very high preloads after assembly. These high preloads require high installation torques, and the poor drive characteristics of hex heads leads to deformation and fracture of the tools. Torx and Torx Plus were similarly created to reduce installation problems incurred with hex, cross-recess (Phillips), and other drive features.

 

[Fabrico]

Thank you for the reply. But, spline drive bolts are not all that commonly used, and seem only to be in special places. The spline has to be a pain to manufacture and also requires special tools to work with. Why the spline drive over the well-proven double hex? Any aerospace specialist?

 

[yates]

Spline-drive heads are no more of a pain to manufacture than 12-point heads (hot heading). In theory, a spline will transmit more torque than a 12-point (have you ever seen a 12-point coupling on a shaft or in a gearbox ?) Common (aerospace) standard spline-head bolts are MS14181, MS21134.

 

[Fabrico]

I find that hard to agree with. A look at military aircraft, jet and rocket engines, reveals that these are only used in critical applications.

The ability to apply more torque leads right back to the original question.

 

[Sparweb]

Bolts are torqued to improve their resistance to fatigue when loads either fluctuate greatly or vibrate. The torque forces the clamped materials to deform slightly, and the bolt stretches, too. Trying to separate the parts now requires a considerable amount of load just to begin to open a gap. In service these parts will not shake free, and the joint is now much more reliable.

When such a joint is very highly loaded, such as in critical places like landing gear, wing struts, engine mounts, the bolt represents a single point where failure can cause catastrophe. The joint therefore demands a fastener that can withstand large loads, making it necessary to use a very high-strength bolt. But given the fact that all of the loads have been upped, and the stresses are so high, the torque on the bolt must also go up for it to be effective.

Around the engine, the temperatures make the situation worse by introducing thermal expansion and contraction into the equation. Again, the parts can either separate in service, or expand against the bolt enough to cause it to break.

Now the question is "how does that bolt get torqued when it is installed". Put an old 12-point socket on a 1/2"-20 bolt hex head and don't expect to get 100 ft-Lb without damaging the hex on the bolt head (or maybe the socket). This doesn't fit the tidy way aerospace people like to do things. It was probably tried, and didn't work. A new head form was devised with a spline to reduce the slippage. Now the bolt can be installed, torqued, checked 100 hours later, removed during a late night overhaul, put back, re-torqued, and removed one last time, without any worry of smashing the points so smooth that you can't get it out again.

BTW, the head is forged at the factory, just like a hex. Getting the shape right is a matter of tooling, but I don't think it would be a radical new process from the factory's point of view. I doubt that it would be a pain to manufacture. I could be corrected on that point.


Steven Fahey, CET

 

[Fabrico]

My comparison, time and again, was to double hex (12-point) aerospace fasteners, not a "six point bolt" and an "old 12 point socket". Nor was the head shape claimed to be a "radical new process".

It makes sense from a repetitive maintenance standpoint, although considering where they are used, I would be skeptical of much re-use.

Again there is inference of possibly being stronger. Well, are they stronger than other fasteners in their class, or are they just the same but used at a higher torque?

Respectfully, nothing so far explains why or when these, obviously more expensive, fasteners are chosen over other high quality aerospace fasteners.

 

[yates]

What do you find hard to agree with ? I did not mention anything about applications for spline-head bolts. I just made a statement about the difficulties of producing spline-heads, as I manufacture bolts.
Both TVP and Sparweb have answered your question. To put it plainly :
1.Spline head bolts allow high installation torque without tool damage.
2.High installation torque allows high joint clamping forces
3.Critical applications often require high joint clamping forces

Therefore, most spline-head bolts around are in high-strength material (220 ksi) since that is another requirement of 3. Using them in other applications would be wasteful, not because of production cost, but because of extra tooling cost for maintenance. (Quite a few spline-head bolts are dismantled for maintenance eg. in landing gear, and are not normally reused).

There is abolutely no difference in strength (for the same class, between a spline-head and any other tension-bolt head (12 point, hex, hex socket cap screw...)
Why do you say 'obviously more expensive ? Spline heads cost the same to produce (in same material) as 12 point heads.

 

[Screwman]

Yates,
Good reply.
The spline drive helps to reduce the possibility of damage to the head or tool when torqued to very high levels. These high levels tend to be associated with high tensile strength (>220ksi) and those locations where there may be significant galling or siezing (high temp locations).
The selection of the drive system is part of the overall engineering of selecting the proper fastener design for a given application. Since they are less common (as are the tools), you would tend to select a spline drive for those locations that require it's capabilities.
SPLINE DRIVE IS NOT BETTER THAN TWELVE POINT; IT JUST PROVIDES DIFFERENT CHARACTERISTICSFOR DIFFERENT APPLICATIONS.
The manufacturing cost for spline will be slightly higher than for twelve point due the lower volumes that the heading tools are purchased, combined with the somewhat lower tool life associated with the forming of the sharp inside corners. It is not a big portion of the overall cost though.

 

[diamondjim]

Screwman,
If the spline drive is not better,
why was it developed? Why has it
not been obsoleted?

 

[yates]

Diamondjim :
Because it's the only drive capable of transmitting high torque without tool or head damage, as stated above. Many other head drives have been developed over the years to transmit higher torques (Torx, Motorq...)whether a particular drive 'survives' or not depends largely on what military and commercial programs it was specified on, rather than mundane considerations of torque transmission, tool availability and wear. And, as Screwman points out, as is always the case in fasteners, costs are closely linked to quantity produced and therefore potential market, rather than material cost, tooling cost....etc.

 

[Fabrico]

Thanks all. Just looking for non-dramatic facts about apples only.

Info presented here suggests that spline and double hex are generally made of the same materials, recieve the same heat treatment, are the same strength, and cost the same to manufacture.

The difference is that splie drives are used upon harder surfaces, torqued to higher levels, and often used in critical, single point load situations.

If this is accurate, and unless there is an obvious sizing difference, it would mean double hex are substantially underutilized and overpriced because they don't do any of these things.

 

[Screwman]

This is a subject that really requires a lot more room than we have here. I have been working in fasteners for 28 years and drive system selection is one of the more involved aspects. It starts off with the fact that there is no single "best" drive system. The best drive system for a particular joint is going to be dependent on the requirements for that joint.
You balance off torque capability, UTS (on recessed heads), removabillity, mass, envelope, headabilty, tool availability, if the drive system is in that customers standards book, fatigue life, etc. to come up with one or move proposals. In a fair number of cases you end up not using the "best" drive because there is a need to commonize drives with other co-located parts for servicability issues or because of other issues that have little to do with engineering.
Saying that spline or 12 point is best is not correct, they are both the optimum solution but for different applications.
The cost aspect is driven by the fact that there are many more 12 point parts manufactured than spline. In OEM volumes I would say that on identical parts comparing 12 point to spline, the spline drive one would be priced about 20% higher due to tooling and manufacturing cost increases.

 

[harrisj]

On a related topic, why were RIBE head bolts invented and what are their pros and cons compared to hex socket heads, splines, Torx etc?

 

[CoryPad]

RIBE is the short name for Richard Bergner Holding GmbH & Co. KG in Germany. Their Fastening Systems unit makes fasteners for many markets, including automotive. I am not aware of any special head bolts they have invented, but they do have many highly engineered fasteners for many applications.

Regards,

Cory

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