By Chuck Lynch
As many of our readers know, AERA has a Tech & Skills Regional Conference program in which AERA members or associate members will host the event. The event usually consists of classroom training, a facility tour, as well as socializing and networking.
Our most recent Tech & Skills Conference was hosted by Point One Manufacturing (P1) in Rogersville, Missouri. P1 is an ultra-modern manufacturing facility, and the team was excited to open their doors to industry folks and show off their facilities and capabilities.
In this article, I am going to share some of the material that Mark Campbell and Shannon Strother presented to the attendees to help better understand the function of a fastener and the challenges that inhibit predictable fastener performance.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2024EPQ4/#p=62
I am going to interject that this article is not about torque to yield fasteners. Many of the rules apply but TTY fasteners operate differently, and I do not want to mislead you. That said, torque to angle does not mean torque to yield, so if you see specifications that call out degrees of rotation, it is simply because it has been determined that degrees of rotation have been deemed the method to achieve proper tension.
What is T=kDP?
T= Torque Input
k= Friction Factor/Coefficient of Friction
D= Diameter of Fastener
P= Tension (Clamp load in Lbs.-Force)
When assembling anything that utilizes fasteners for retention, the goal is clamp force. Without the proper clamp force, at all times, things tend to come apart and that is failure. So, to get clamp load, we need to ensure that we have a process that allows us to create a proper preload.
It’s worth saying again here that Torque is not Clamp Load.
In this case, Torque input is the measurement of the energy required to tighten (create PRELOAD on) the fastener. When you stop applying Torque to the fastener, the fastener is now in tension and the result is Clamp Load on the assembly. The only value in the T=kDP equation that we can attempt to control is K (friction). Torque cannot be adjusted per fastener, Diameter of the fastener does not change, and Tension (Clamp Load) is the result of the equation.
What is the number one hurdle faced when trying to generate clamp load? The “k Factor” is that hurdle. Only 10% of the total torque input is used to tighten (stretch) the fastener. The balance of the input is consumed by these primary friction points.
• Threads 35% +/-
• Undersize of nut/bolt flange 55% +/-
To combat some of these losses, there has been much effort in developing lubricants that modify and/or seek to control the k Factor more tightly. As Lake Speed Jr always likes to preach, you need the right material in the right place in the right amount, at the right time. Therefore, care must be taken to not adversely impact the performance by not following Lake’s rule.
To that point, there are risks involved with modifying the k Factor. If you have a material that is too slick you could cause excessive bore distortion, stripped threads, broken or cracked castings, excessive crush on gaskets and of course damaged fasteners. If you have an inferior material, you may never achieve the rotation necessary to generate the proper preload/clamp force.
In Figure 3, you not only see that there are vast differences in clamp load, but you also observe that the degrees of rotation achieved by this torque input is quite different for the three examples. This is another reason that many tightening strategies have moved to torque to angle (TTA). Regardless of the type of lubricant, or if no lubricant exists at all, angle is always angle. That does not mean that there is not potential for galling and fretting in the threads or under-head of the fastener, but the fastener will not be over-rotating and causing the variation in the resulting clamp load that you see illustrated.
Another particularly important factor which may often be overlooked by engine builders is the extraordinary effect that materials may have on clamp load repeatability or variability. In Figure 4, we see the data showing the same fasteners tightened many times using identical torque inputs. The disparity in clamp load between one stud and another in your assembly may be much greater than you imagine. Once again, this may result in two main studs or head studs being installed right next to each other – but having radically different clamp loads applied to the engine. This can cause unpredictable bore distortion, excessively or inadequately crushed gaskets, and of course some random damaged fasteners.
So, just what do some of these loads look like in relatable terms? In Figure 4, you will see that the 1/2″ (12.7mm) diameter stud has a theoretical calculated clamp force of 16,044 lbs.-force.
Have you ever wondered what 16,000 lbs. looks like besides a number on paper? Now imagine that I have ten main studs and not just one. The clamp forces required to keep an internal combustion held together are staggering.
As you can see, ensuring proper clamp force can be elusive if you do not follow the manufacturers recommended tightening procedures and lubricant recommendations.
Here are a few check boxes to help you ensure the best possible results.
- Ensure that the threads of the fasteners and in the receiving casting are clean and not damaged.
- Use the manufacturer’s recommended lubricant. (If you have a lubricant that you always use and want to try it with P1 products, call them and they can evaluate and add the results to their library if they do not already have that data on file.)
- Maintain your torque wrenches. Ensure current calibration. Wrong information is worse than no information in many instances.
- Take your time when torquing. You are not a windmill, and fasteners are not drill bits. Vast disparity in achieved clamp load is one cause of bolts backing out or gasket failure due to poor load distribution.
- If there is a tightening specification that has a stretch measurement or a torque to angle measurements, follow the recommendations and do not try to find a torque spec to make your job easier. We have just defined why a torque value alone may not be telling of the whole story.
That last check box may seem like a no brainer to some, but we have had that call on the tech line plenty of times. There are many things that are out of an engine builders’ control that they get blamed for. Proper tightening of fasteners is one thing that is definitely in the control of the engine builder.
If you ever find yourself in the Rogersville/Springfield Missouri area and would like to see a world class manufacturer, stop by P1. You will be impressed by products, processes, and facilities, but most importantly the team that is P1.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2024EPQ4/#p=62