Conical Spring Wedge Lock Washer Pairs

It is well documented that loosening of a bolted assembly takes place when the fastened bolt/nut rotates "spontaneously", and often the slightest rotation of only a few degrees is sufficient for a bolted connection to lose all its preload. This rotational loosening is exacerbated when any part of the bolted assembly experiences slackening, the collective term for the various causes of non-rotational loosening.

Our Conical Wedge Lock Washer Pairs address both the rotational and non-rotational causes for loss of preload in a well-designed bolted connection, firstly (as the Safety Washer does) it uses CAM geometry and contact GRIPS to deal with spontaneous bolt loosening from shocks, vibrations, and dynamic loading and secondly, (which is where our improvement focuses) it addresses slackening due to settlement, creep, differential thermal expansion, relaxation and yielding, through the introduction of a Conical Disc Spring to the overall shape.

This 1-pager provides a summary of target preload, and required torque , by bolt size and bolt class, for a range of k-factors.

Theory behind fastener self-loosening

Bolted joints rely on the clamping force, also called preload, that results from the tightening torque. The fastening will not come loose if the clamping force acting on the bolts is sufficient to overcome the transverse force and creates sufficient friction grip to prevent transverse movement between the clamped parts.

The tightening process.

To better understand the phenomenon of self-loosening, we first have to study the tightening process. A tightening torque M_A is necessary to fasten a bolt. It is the combination of the torques related to the friction under the head M_K, the friction in the threads M_G and the friction in thread pitch M_TP:

Consider the fact that, in some instances, only about 10% of the tightening torque leads to joint preload M_TP (Schematic 1). If friction under the head MK or friction in the threads MG is increased by only 5%, this can lead to a reduction in preload of 50%

Torque Calculaions

The torques related to the friction under the head M_K, the friction in the threads M_G and the friction in thread pitch M_TP are calculated using the following formulae

Where:

• F_V = the preload in the bolt
• μ_K = friction under the head of the bolt
• μ_G = friction in the thread
• P = thread pitch
• D_KM = effective diameter for the friction in the bolt head or nut-bearing area
• d₂ = thread pitch diameter

In a bolted joint, the thread pitch friction M_TP is less than the combined friction in the thread M_G and under the head M_K. This is called self-locking. The tensile force generated by the elongation of the bolt shaft and by the force of compression generated in the objects being tightened remains balanced::

Spontaneous Loosening

In practice, most bolted joints experience influence from the surrounding environment. This can lead to a change in the balance (the equation above) and a spontaneous decrease in the preload results. To loosen a bolted joint, the moment M_L is necessary:

The pitch torque M_TP works to unfasten the bolt. This is because of the helix angle of the thread pitch, also known as the internal off-torque. Thus, when the internal off-torque is larger than the retention moments M_K + M_G, rotational self-loosening will take place, i.e. when:

Marginal Slip

The transverse movement required to overcome the frictional resisting force, also called ‘theoretical limiting displacement’ SG_th or ‘marginal slip’, can be calculated as follows:

Where:

• SG_th = theoretical limiting displacement
• F_V = the preload on the bolt
• μ_K = friction under the head of the bolt
• L_K = clamping length of the bolt
• E = Young’s modulus of the bolt
• I = second moment of area of the bolt

The formula is based on a model of the bolt as a rigid beam. The reset forces due to slip in the connection have to be smaller than the retention due to friction.

This equation provides us with some insight on how to prevent spontaneous loosening in a bolted connection. To realize a displacement under the head of the bolt the motion between the two clamped plates has to exceed a specific amount, the marginal slip. So by increasing the marginal slip value, we secure the bolted connection, by:

• increasing the preload
• raising the friction coefficient under the head of the bolt
• increasing the bolt clamping length
• reducing the second moment of area of the bolt

Ironically, all of these contribute to other phenomena, which lead to loss of preload. The first of these is settlement.

Settlement

Settlement takes place on the contact surfaces between the various assembly parts (red arrows above) and is easily observed by indentations. It is caused by localised plastic deformation. Surfaces may appear to be smooth but when sufficiently magnified, jagged peaks are evident (these are the same structures that Shot Peening targets as they are also the source of fatigue crack propagation). They are called asperities.

The contact area between opposing asperities can be substantially less than the apparent area. It is these asperity contacts that deform and flatten. The loading to cause this is substantially less than the load needed to plastically deform the entire apparent contact area.

Stress-Relaxation

Stress-Relaxation is the decrease of preload, experienced over time, with no visible physical change to the bolt. This is a form of creep and occurs when high stresses present in a bolt are relieved over time, most notably, at elevated temperatures (and this can be a very useful process, in fact an essential one in reducing fatigue cracking in other instances of metal working). Unlike with settlement or creep, the clamp length does not change, which makes it more difficult to detect.

The drop of preload directly following the tightening procedure is mainly attributed to settlement and elastic recovery. Subsequent loss of preload follows a power function with time. Retightening of bolts can reduce the preload loss due to relaxation, but the preload loss directly following the tightening procedure prevails, especially if tightening is done at a fast rate, for example when using a hydraulic torque wrench or a specialised tool.

Creep

Creep (sometimes called cold flow) is a property of materials that results in progressive deformation and is a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point. The rate of creep deformation is a function of the material's inherent mechanical properties, exposure time, exposure temperature and the applied load.

When slackening occurs for any reason, the typical wedge geometry will not maintain preload and keep the connection secure. This is why we have taken our years of experience in Conical Disc Spring design and manufacture and applied these insights to the issue of slackening as a cause of preload loss.

We have explained the sources and cause of slackening in a bolted connection above and explained why, when slackening occurs for any reason, the typical wedge geometry will not maintain preload and keep the connection secure. This is why we have taken our years of experience in Conical Disc Spring design and manufacture and applied these insights to the issue of slackening as a cause of preload loss.

Evolution of the Wedge Lock Safety Washer

So what is the result of combining the geometry of a safety wedge lock washer with the form of a conical disc spring?

Addressing causes of Preload Loss

Our improvement on the Wedge Locking Safety Washer addresses the primary reasons for loss of preload in a well designed bolted connection, firstly (as the Safety Washer does) it deals with spontaneous bolt loosening from shocks, vibrations, and dynamic loading, snd secondly, (which is where our improvement focuses) it addresses slackening due to settlement, creep, and relaxation. Unfortuately, bad design will always be difficult to negate.

In the photograph below, notice how when the Conical Wedge Lock Washers are stacked on each other, they are not flat but instead shaped so that each forms a dome or cone shape. Because they are manuafctured from Spring Steel and specifically hardened and tempered, when the pair are flattened against each other, this will act as a spring. As the bolt assembly relaxes with time, the relaxation is taken up by this Spring effect. This counteracts the second primary reason why bolted connections fail and loosen.

We avoid issues with bolt assembly fatigue and Slackening by using Spring Steel as our default base metal (never Mild Steel) as this is essential for the properties of the conical disc spring which we employ to counter any relaxation. (Note: we have been designing and manuafcturing DIN 2093 and more recently DIN-EN 16983 conical disc springs for more than 3 decades and have made all the mistakes there are to be made in learning about the various alloys, working them and heat treating them!). This disc spring effect then works in combination with a CAM working surface that imposes an incremental retarding force/drag as any rotational movement will work against the gradient of the CAM as the nut loosens (the so-called "wedge").

Our Conical Wedge Lock Washers are used in mating pairs, where the two inner bonding surfaces will interlock together because of the CAM surface symmetry. System movement is only possible between these two inside surfaces and because of this geometry, readjustment of the load force happens automatically as the inside surfaces move in relation to each other. In this manner, it is a geometry driven pre-load force rather than just friction alone that ensures a secure connection.

Some Technical Detail

Conical Wedge Lock Washers are manufactured from specially quenched and tempered spring steel, and have the following characteristics:

• The angle of the inner surface wedges is greater than the pitch of the threading bolt. This ensures that any longitudinal movement of the connection due to movement up the thread is less than the travel that might occur as the two inner surfaces work across each other.
• The outside surfaces (we call this the GRIP) of the two mating wedge lock washers are manufactured in such a way that their coefficient of friction is greater than the coefficient of friction between the two inner working surfaces (we call these the CAM). This ensures that any movement caused by dynamic stresses occurs in the region between the inner wedge shaped surfaces, rather than on any outer surface.
• The surface hardness of both the outer and inner sides of the wedge lock washers is harder than that of high tensile bolts (8.8, 10.9 and 12.9) and because we use wear resistance Spring Steel hardened to Rockwell C of 50, specifically designed for hard-wearing and abrasive conditions, the surfaces of the wedge lock washer remain true.
• In highly corrosive applications or where non-magnetic material is essential, we subject the stainless steel to work hardening during manufacture, and select stainless steel which has the attribute of getting harder as the material is worked. We will also use INCONEL or AISI 422 X22CrMoV12-1

The picture below of an M20 pair after 3 months in situ, illustrates the inner CAM "wedges"(left hand side) and outer GRIP (right hand side) surfaces.

Available Sizes

We manufacture Conical Wedge Lock Washer Pairs in both metric and UNC bolt sizes but only from M12 and 1/2 inch up to M125 and 5 inch. The complete listing is available here. As mentioned we do not (yet) cater for smaller bolt sizes, however if you need anything larger or something that is not on our published size list (and you think should be), we are only too happy to accommodate you as best we can.

We also manufacture Concial Wedge Lock Washer Pair sizes that complement our range of DIN6796 Belleville Washes, used specifically for static Bolted Connections, these have a greater outer diameter to accommodate the Belleville Washer.

Available sizes (by bolt size) are listed along with our other solutions for Bolted Connections.
It is useful to note that we provide you with the applied load the washer pair will excert on the connection when fully flattened, how much the pair must be compressed to be flattened (in mm), and the stress value that most impacts setting, which we are avoiding, so that the Spring effect is maintained for sustained periods, and if loosened the Conical Wedge Lock Washer Pair can be reused.

Testing

As you might appreciate, the majority of our development time has been allocated towards rigourous testing. This is a non-trivial exercise, and we have devoted a whole section to our customers on all things related to testing.

We strongly encourage you invest some time reviewing our testing. Here you will find everything you need to have some peace of mind in our solutions.