Titanium as a fastener material

Thursday, November 10, 2011

Tags :  advanced-materials

advanced-metalsTitanium is banned in the current Formula One engine regulations from being used for threaded fasteners, despite its attractive attributes for such components. The rules specify that threaded fasteners must be made from alloys based on one of three elements - iron, cobalt or nickel - and this is planned to be carried forward for the new V6 turbo engines we will see in use from 2014 onwards. It should be noted though that there is no similar regulation governing the use of titanium fasteners on the chassis.

Besides titanium's obviously attractive property of low density, its elastic modulus is the other property that makes it a good candidate material for fasteners, both of the internally and externally threaded varieties. The use of nuts with lower modulus than the male fastener is known to reduce the stress concentration effect at the first thread, and improves the distribution of load over the length of the engaged threads. Where high-modulus materials are specified for both internal and external threads, one way to achieve the same effect is to use combinations of male and female parts with very slightly differing thread pitches.

When considering the design of a stud or bolt used for cyclically loaded fastener, it is important to consider both the fastener stiffness and the stiffness of the parts being clamped. A simple formula involving these quantities dictates how the service load is shared between the unloading of the joint, and the extra load borne by the fastener. This relationship has been covered in one of the early RET Monitor articles on fasteners and in a past article in Race Engine Technology magazine*.

The smaller the stiffness of the fastener is compared to the stiffness of the joint, the less of the service load that is borne by the fastener. Ideally, what we want from a fastener material is for it to be strong - fatigue strength is the significant strength in a cyclically loaded fastener - and to have low stiffness. Titanium can score well here, and its lightness is a bonus, although that shouldn't come as a surprise. Most metallic materials fall within a pretty narrow range of specific modulus (modulus divided by density) and so any material with a low modulus is likely to have a low density. There are some notable exceptions to this 'rule of thumb', such as beryllium, but most common aluminium, magnesium, titanium and steel alloys we are likely to commonly use have very similar specific modulus values.


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There are some technical problems though with the use of titanium as a male fastener. Its tendency to gall at low levels of load when sliding means it needs to be installed with special grease, or needs to have its surface treated to prevent the problem, especially where it is used in conjunction with a titanium nut. However, the problem is far from insurmountable, and racing motorcycles of 20 years ago were festooned with such fasteners throughout the engine and chassis, as are many racecars, motorcycles and boats today. It seems strange that they are now outlawed in bespoke race engines at the highest levels of motorsport, but are affordable to low-budget racers.

*Race Engine Technology, issue 41, September/October 2009

Fig. 1 - Was motorcycling in the early 1990s a richer sport than Formula One is today? Titanium fasteners are banned from Formula One engines, but were affordable for endurance racers such as this beautiful Honda RVF750

Written by Wayne Ward

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Comments

The issue with titanium threaded fasteners is not just their propensity to gall, it also has to do with titanium's notch sensitivity.  A highly stressed titanium part with any surface defects or scratches will readily fracture.

Titanium threaded fasteners only work well for installations where the joint never comes apart.  Otherwise, their inherent galling characteristics makes Ti fasteners a real pain.