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Engine Building Fundamentals: Bolt Clamping Force

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Bolt Clamping Force

07.22

00:00 - There's huge amount of force being created inside a running engine.
00:04 Combustion pressure is trying to lift the cylinder head from the engine block.
00:07 The crank shaft is being pushed out the bottom of the block.
00:11 And the piston is constantly cycling between trying to pull the conrad apart and trying to compress it together.
00:18 With this in mind, one of the most critical aspects of maintaining a reliable engine and managing all of these forces is the fasteners used inside the engine.
00:29 When I'm talking about fasteners I'm referring to anything that holds two components together.
00:35 In our engine application we typically going to be considering what's known as a tension joint.
00:41 Where we have two surfaces clamped together by a fastener.
00:45 For an example, let's consider the joint where the cylinder head is bolted to the block.
00:51 In this example we have applied forces from compression and then combustion, that are trying to lift the cylinder head off the engine block.
00:59 These forces are opposed by the clamping force provided by the fastener.
01:06 In order to prevent the head from lifting off the block the clamping force must be equal or greater to the applied forces from compression and combustion.
01:16 The part that's easy to overlook is that a fastener is really nothing more than a spring.
01:22 And in order to achieve a clamping force we need to actually stretch the fastener.
01:27 This is what we're doing when we're tightening a bolt.
01:31 Simply stretching it to provide a clamping force.
01:34 So now we have a better understanding of the basics of how a fastener holds our parts together, we're going to consider what actually happens to material of the fastener as we tighten it and it begins to stretch.
01:48 It's worth noting here that you don't need to be a mechanical engineer to tighten a bolt.
01:53 However, it's useful to understand what's going on with the material as we tighten the fastener and you'll also be able to better understand some of the terms you'll commonly hear.
02:05 To explain what happens lets take a sample of material and we'll place it in a tension compression testing machine.
02:12 This machine is used to test the properties of a material and it applies an axial load that stretches the sample material while measuring the elongation.
02:22 Knowing the original length and the cross section of the material the stress and strain can be calculated.
02:29 We can then graph these and produce what's known as a stress-strain diagram.
02:34 Which might look something like this.
02:37 Stress is on the vertical axis and this is defined as the force being applied to the material divided by the cross section of the area.
02:46 To keep it simple you can consider the stress to be the clamping force that the fastener is supplying.
02:53 Strain on the other hand is the amount of elongation or stretch that the material is subjected to.
03:00 Now we know what these terms mean we can consider a few key points on the stress-strain diagram.
03:07 As we first begin applying force and stretching the material we can see that the line on the graph is linear up to the point labelled the proportional limit.
03:17 What this means is that in this area of the graph the elongation is directly proportional to the force being applied.
03:26 As we move past the proportional limit we can see that the shape of the graph begins to curve and we reach a point labelled the elastic limit.
03:36 Provided that we're working within this region of the graph when the force is removed from the fastener it will return to its original length and won't be permanently deformed.
03:47 To put this another way this is the maximum amount of clamping force the material can develop without being permanently deformed or stretched.
03:57 As we move past the elastic limit we reach the yield point.
04:01 Which is a point where the material will show us significant elongation or stretch without an appreciable increase in load or clamping force.
04:11 If we continue to stretch the material past the yield point we find that the load or clamping force begins to increase again up to the point labelled ultimate strength.
04:23 This is the maximum amount of clamping force that the particular material can provide.
04:29 And if we continue to apply more stretch to the material the stress will begin to drop until we reach the rupture strength where the material actually fails.
04:40 The region of the graph to the left of the yield point is referred to as the elastic region.
04:47 While the area to the right of the yield point is referred to as the plastic region.
04:53 If we're operating in the plastic region of the graph the fastener will be permanently deformed.
04:59 Which means that when we remove force from it it will remain stretched.
05:05 So now you should have a better understanding of what happens as we tighten a fastener and there's a few takeaways from this information.
05:13 Many fasteners and particularly those used by OE manufacturers are referred to as torque to yield bolts.
05:21 These fasteners are designed to be stretched past the yield point as their name implies and hence should be replaced after use as they will be permanently stretched.
05:33 The torque to yield bolt will initially be tightened to a specific torque that will have the bolt within the elastic region of the graph.
05:42 Beyond the initial torque specification the bolt we then be tightened even further by a specific angle perhaps 90 degrees for example.
05:51 The specified angle is calculated to put the material somewhere between the yield point and the ultimate strength of the material.
06:00 Providing more clamping load than if the fastener was operating in the elastic region.
06:06 Of course when the fastener is removed it will be permanently stretched though which is why it must be replaced.
06:12 If we're dealing on the other hand with a fastener that only uses a torque specification, then this fastener is designed to work within the elastic region of the graph and it can be reused as it won't be permanently stretched when it's installed correctly.
06:28 Many of the aftermarket fasteners that we use in the performance engine building industry from the likes of ARP are designed to work within the elastic range of the material and are reusable.
06:40 Despite working within the elastic range the material is usually superior to that used in OE fasteners and hence the fastener can provide superior clamping force as a result even while still operating within the elastic region.
06:57 So from this module you need to understand what happens to the fastener material as the fastener is tightened.
07:04 You need to understand the difference between the plastic and elastic regions of operation.