Torque to yield; also commonly referred to as angular torque is used in many of today’s modern motor vehicles, predominantly for cylinder head bolts but also main bearing caps and suspension components.
Angular torque has been used for many decades throughout Europe in the manufacture of assemblies using composite materials such as cast iron cylinder blocks and cast aluminium cylinder heads. Angular torque came to prominence in Australia when manufacturers started to specify this method dating back to the EA Falcon and Holden Gemini. This was a game changer in the design and manufacture of products that required angular torque specifications and the torqueing of head bolts to be approached from a different perspective. The tricky bit with composite materials is the difference in the co efficient of linier expansion between iron and aluminium with aluminium expanding and contracting to much greater proportions and at a much faster rate which led to problems such as how to keep a proper head gasket seal when the engine is cold on start up and when at correct running temperatures.
With a conventional head bolt, a torque specification is applied to create enough stretch in the bolt to provide sufficient clamp load to keep the gasket in place throughout the operating temperature range of the engine. Torque loads are specified to the conventional bolts to give enough stretch to maintain the correct clamp load but most importantly, still remain within the elastic limit of the bolt allowing the bolt to return to its static dimensions and reused. With the bolt in situ trying to return to its static length is what provides the clamp load on the gasket. A great example of this bolt movement was quite clearly evident in the early Sigma cylinder heads which suffered from a softening of the cylinder head when overheated allowing the bolt to return to its original length by driving the bolt head washer into the surface of the cylinder head which in turn caused a major reduction in clamp load and consequently gasket sealing capacity and failure.
Torque to yield or angular torque is a different story altogether and different processes are required. Compared to conventional type bolts, torque to yield head bolts offer the engine manufacturer a number of advantages:
- greater flexibility of design
- reductions in component costs
- more accurate torque specifications
Engine design utilising torque to yield head bolts require fewer bolts to achieve the desired clamp loads than those of conventional bolts. With fewer bolts the engine manufacturer has more flexibility in the cylinder head and block design as well as reducing the cost of the engine.
Accuracy of the torque application to the bolt is also more easily controlled with torque to yield as the bolt is not so much subjected to friction loss as a conventional style of head bolt. When applying the load with a torque wrench directly to the bolt, the friction loss can be as great as 90% with 45% – 55% being head friction and 35% – 45% being thread friction leaving a clamp force as little as 10% of the applied torque.
While torque to yield head bolts are cost effective to the manufacturer there are something that the engine repairer will need to be aware off. In most cases it is not a requirement to replace conventional head bolts unless they are damaged i.e. damaged threads, the bolt head was rounded off, and the shank is severely corroded or pitted. Conventional head bolts simply did not wear out. Torque to yield head bolts however, by the very nature of their design do deteriorate and should never be reused.
When a bolt comes under load, it exhibits four main phases: the elastic phase, the plastic phase, the yield point and the shear point. In the elastic phase, a bolt will stretch under tension but will return to its original condition when the load is removed. As we continue to apply the load the bolt reaches the plastic phase from which it cannot recover to its original length and is now permanently stretched. The point that separates between the elastic phase and the plastic phase is called the yield point. We have all experienced the plastic phase when a distinct softening of the load occurs. This is known as the oh moment that you know you are in trouble. To continue to apply the load, the shear point will be reached and the bolt breaks.
As with conventional bolts, torque to yield bolts are tightened in a series of stages and in sequence. Torque to yield bolts are generally tightened in sequence to what is known as a snug torque value. This snug value is to ensure that the component, in this case say a cylinder head is firmly located on the block. Correct sequence tightening is essential to eliminate distortion. Torque is further applied in sequence through a series of motions only this time instead of torque load being measured, angle of rotation is measured. The manufacturer will have specified what load is required and this specification could look as follows:
- Tighten in stages and sequence to 40Nm
- Tighten in sequence to 90degrees
- Tighten in sequence a further 90 Degrees
Measuring rotation in degrees instead of an applied torque figure ensures that friction does not overcome the tightening process and that an even clamp load is easily achieved.
Different to conventional bolts, torque to yield head bolts are tightened beyond their elastic range, past their yield point: i.e. past the point from which the bolt material can recover to its original length: and into the plastic phase of the bolt material.
The bolt is permanently stretched and for this reason should not be reused. The reliability of these bolts once stretched is greatly reduced, if they are reused, they are permanently stretched further a second or third time. It is for this reason, you should never reuse a torque to yield bolt and always follow the manufacturers specifications.
For more information on the correct tools for Angular Torque – Torque to Yield please contact Warren and Brown about their Electronic Angular Torque Wrenches and Angular Torque Gauges.