Fasteners
The table consists some of the fasteners' technologies claimed to be superior to conventional nuts and bolts.
Prevailing torque
The prevailing torque is the torque required to run a nut down a thread before engagement with the joint surface.  Several of the locking techniques rely on a distortion of the nut thread or shape to create an interference fit. Metal or nonmetal inserts (plugs, strips, or collars) that plastically deform are also used to create an interference fit. Very popular design in this category are nylon insert fasteners, promoted as anti-vibrational. However, tests show that they fail in vibration at the same number of cycles as conventional fasteners.
Serrations
The integrally formed teeth or serrations on the bearing surfaces are shaped and directed to bite into the surface of the member being joined when tightened.
Wedges
When pitch of a thread is smaller than pitch of wedges, turning the fastener on the wedges, in unscrewing direction, wedges the thread.
Tension sensor
This fastener is not different from the conventional one with one exception; it allows sensing tension in the joint. The precious feature comes at a cost, as each joint needs to be assessed, preload calculated and the fastener has to be laboratory calibrated. The six stage process was filmed and could be watched at http://www.rotabolt.co.uk/ by pressing "See how it works"
Multi-Jackbolt
Multi-Jackbolt.
Delnut
Delnut is not an improvement; it is a Revolution in fasteners!
Delnut represents a complete, uncompromised fastening solution.
It is an ultimate High-Performance fastener.
 
Delnut was originally developed to address fastening problems of equipment exposed to vibration, fluctuating and thermal loads, where no effective solution to unscrewing and fatigue failure existed.

 
The tapered thread imposes a prevailing torque on the fastener. The minor diameter is tapered to interfere on the last three or four threads of a nut.

 
A variation of the technology is a serrated washer.


Wedges formed integrally on the nut.
The proper tension in the joint is indicated by changing color.

Anti-vibration lock
Vibration loosening of fasteners occurs in two steps.
First is due to a cyclic plastic deformation of the materials.
The second stage is characterized by the backing off of the nut.
To prevent vibration induced loosening a locking features, like wedges or ratchets are required.
Junker machine tests, according to DIN 65151, indicate that all the other preventive measures fail.
No
Prevailing torque nuts could not prevent loosening of the joint. In Junker machine vibration tests with M10x35 bolts, at 25 kN, 12.5 Hz, +_1.0 mm amplitude, the final preload dropped to zero after 200 cycles for nylon insert and 1,000 cycles for all metallic nuts. However, there remained a prevailing torque, which prevented the fasteners from failing apart.
Torquing these fasteners over a long thread travel could damage the locking feature.
They are not reusable, because removal often damages the threads or the plastic patch.
Due to high resistance during assembly, they are prone to galling and require more effort to ratchet down using special tools.
Vibration tasting indicates that plastic patch prevailing torque fasteners do not present any meaningful improvement over conventional fasteners.
No
Tests show that serrated fasteners tend to withstand vibration only marginally better than prevailing torque fasteners. Therefore serrations do not constitute positive anti-vibration lock.
Yes
Wedges prevent unscrewing by wedging the thread, so unlike friction, they constitute positive mechanical barrier. Wedge-fasteners have been patented over a century ago, but only recently have become more popular. When intelligently incorporated into a fastener, wedges are capable of greatly improving anti-vibration fastening.
No
The gap between the crest of the male and female threads leads to vibration-induced thread loosening.
Devoid of any locking mechanism, these fasteners are not suitable for vibrating environment.
No
In case of Multi-Jackbolt, there is an unresolved problem of unscrewing, not just of the main fastener, but also of the jackbolts.
Yes
Unlike any other fastener, the Delnut is totally friction independent, thereby operational forces, vibration or shocks will not unscrew it. Forces that cause vibration and make other fasteners unscrew, induce Delnut retightening, while other so-called self tightening fasteners default to even bigger slack.
Delnut incorporates wedging system for dynamic screw locking and ratcheting system that locks up to predetermined torque.
Fatigue resistance
A threaded fastener joint can withstand fluctuating loads better (less prone to fatigue) when the force capable of its separation increases, because external loads variation before separation causes maximum of 5% to 10% increase in bolt�s tension, depending of stiffness ratio of bolt and the joined parts.
This can be achieved by:
1. Lowering stiffness of the bolt,
2. Preloading to near the yield point of the bolt and
3. Even distribution of loads amongst the engaged threads.
No
Prevailing torque fasteners are often marketed as having superior anti-fatigue properties.
This is due to more even distribution of loads amongst the engaged threads.
However, increased torsional stress prevents proper preloading, which offsets the first mentioned advantage, resulting in the accelerated loss of preload and subsequent failure due to fatigue.
No
Serrated fasteners must embed themselves into a work-piece and, in case of washers, also into the threaded fastener. However, the embedment on the surface against which rotation happens cannot occur during the preloading. Sinking serrations cause a very substantial loss of preload, leading to fatigue failure.
No
Wedge-washers contribute substantially towards shortening of the fatigue life of fasteners primarily due to substantial preload loss caused by embedding of the ridges and difficulty with setting the proper preload. Stress concentration at the first few engaged threads causes an additionl fatigue risk problem.
Yes
Tension sensors can be preloaded very accurately, so they have much better fatigue life than conventional fasteners.
No
Multi-Jackbolt is particularly prone to fatigue due to severe losses of preload. Most of the loss is due to creep, which affects highly stressed jackbolts. Loss of preload also occurs in the main thread, because there is no circular movement between threads during preloading, which tends to eliminate surface irregularities.
Yes
Delnut is trully fatigue resistant fastener.
Such outcome is achieved in three easy steps:
1. The resilient washer boosts bolt elasticity by lowering resultant stiffness of the clamping parts (bolt + washer). As the joint diagram indicates, the washer carries lion share of the external loads.Only small portion, Feb, of the external load Fe rests on the bolt, therefore the effect of the fatigue loading is minimal.
2. Improved angle control method of preloading, according to incorporated scale, combined with torsional stress relief between consecutive ratchets allows close to yield loading.
3. The elastic nut, which works partially in compression and partially in tension, distributes loads amongst the engaged threads evenly.
The Joint diagram shows that
Friction independance
Fasteners traditionally rely on friction to prevent unscrewing. This friction, however, constitutes impediment to proper preloading, because 90% of the tightening torque is consumed to overcome it. Only 10% of the torque loads the joint.
The diagram shows that most energy delivered during preloading is lost (converted to heat).
Non-reliance on friction can have enormous impact on the threaded fasteners use.
- It lifts the limit, of around one inch, from the size of fasteners, which could be torqued without use of power tools. The new limit will depend on friction lowering techniques employed. Savings here could be very substantial.
- It allows wide spread substitution of metal fasteners with plastics, ceramics and other materials that exhibit low friction, which are cheap, light, non-magnetic and resist corrosion.
No
The idea behind the prevailing torque fasteners is increased friction on the thread to resist unscrewing.
No
Until the serrations sink into the opposite material of the joint, these fasteners rely on combination of friction and serrations to prevent unscrewing.
No
Until the ridges sink into the opposite material of the joint, these fasteners rely on combination of friction and ridges to prevent unscrewing. Since tightening of the joint takes place on ridges, lowering friction there would defy the purpose of the ridges.
No
Tension sensors rely on thread friction and under the head friction to prevent unscrewing.
No
Both the main fastener and the jackbolts are friction dependent.
Yes
Delnut is totally friction independent, meaning, it does not rely on friction to prevent unscrewing and is preloaded according to the scale, which is friction independent. Consequently, Delnut employs a variety of techniques to lower friction on the thread and other internal surfaces. Most of the torque applied to Delnut loads the joint, resulting in many times as large fasteners being torqued manually as possible with conventional fasteners.
Even stress distribution
When a joint is assembled with fasteners, other than Delnut, and tightened, the bolt is normally loaded in tension and the nut in compression. The bolt thus stretches axially, causing an increase in pitch of the male thread; and the nut compresses, resulting in a reduction in pitch of the female thread.
The consequence of these two actions is that the threads near the loaded nut face are forced into much tighter contact than the remainder; resulting in a concentration of load, and therefore stress in the thread, in this region. Typically, 73% of the load is concentrated on the first three threads.
The variation of normal stress over a cross-section adjacent to the nut face ( as sketched ) is dominated by stress concentration induced by the thread root, so it is hardly surprising that bolt fractures usually occur in the exposed threads close to the nut face.
Another failure mechanism here is stripping of the nut threads, which is essentially shear failure of the nut material on the cylindrical surface at the thread major diameter.
Yes
There is a great verity of prevailing torque fasteners on the market. Some of the metal prevailing torque fasteners rely on at least partial interference fit between the external and internal thread. Their manufacturers claim better distribution of loading along the engaged parts.
No
Serrated fasteners do not distribute stresses any better than conventional fasteners.
No
Wedges neither improve nor deteriorate distribution of stresses.
No
No improvement here over conventional fasteners.
No
Multi-Jackbolt does not have any feature, which would promote even stress distribution. Both installation and removal require adherence to strict procedures as not to over-compress individual jackbolts.
Yes
The elastic nut, which works partially in compression and partially in tension, distributes loads amongst the engaged threads evenly. This feature minimizes formation of creep.
Stresses in conventional fasteners are represented by the black line and in Delnut blue.
Self-centering
Angularity
Fasteners' self-centering feature is crucially important for proper assembly and operation of the joint. It prevents bending of the stud during torquing-up and its interference with the hole. Bending interposes additional tension on the stud, which is not passed to the joint. Similarly, the hole interference causes some of the tension will not end up as clamping force between joint members. It will also cause increased frictional losses, torsional loading of the stud resulting in lower yield point of the stud, as well as increased residual torsional stress.
Work-piece angularity, if not compensated for, can dramatically shorten fatigue life of the fastener.
No
No self-centring feature incorporated or work-piece angularity addressed.
No
No self-centring feature incorporated or work-piece angularity addressed.
No
No self-centring feature incorporated or work-piece angularity addressed.
No
No self-centring feature incorporated or work-piece angularity addressed.
No
No self-centring feature incorporated or work-piece angularity addressed.
Yes
Delnut does not torsionally stresses bolts as much as other nuts do. This is due to self-centering action and because torsional stresses are relieved between ratchets.
Angularity of the work-piece affects the fatigue life of the fasteners, therefore angularity should be corrected prior fastener installation. However, dish shaped Delnut has a capacity to compensate small angularities.
Accurate pre-loading
Standard wrench +/- 35%
Torque Wrench+/- 25%
Angle Torquing+/- 15%
Indicating Washer+/- 15%
Turn of nut+/- 10%
Ultrasonics+/- 8%
Strain Gauges+/- 2%
An application of a tightening torque to a fastener causes build-up of a tension in the joint. Proper preload is understood to be tension in the joint of around 85% of the proof strength of bolt.
The importance of proper preloading cannot be overemphasized. Improper preload can cause a multitude of failures, including static, shear, overload on impact, fatigue and locking mechanism failure. Selection of preloading method, which determines accuracy of preloading, is vitally important.
Torque-wrenches should be avoided due to very low accuracy.
No
Preloading Torque [Nm] Preload [kN]
Std nut 25 28
Serrated nut 25 20
Serrated washer 25 12
Prevailing torque and serrated fasteners require much higher torques than conventional fasteners to produce similar preload. They suffer from substantially increased tortional stress during preloading. As a consequence yield point can be reached well before proper preload is achieved.
It constitutes barrier to proper fastener preloading.
Prevailing torque fasteners suffer from reduced preload, which can be as much as 16%.
No
Serrated fasteners are more difficult to preload than conventional once. See under prevailing toque fasteners for further details.
No
In case of wedge-washers, tightening takes place on ridges. Manufacturers recommend 20% higher torque to be applied than with the use of plain washers. Preload scatter is substantial.
Yes
Tension sensors are brilliant at accurate pre-loading. They are really laboratory instruments, which are individually laboratory calibrated.
No
Difficulty of proper preloading arises here from the fact that the jackbolts, which form part of the main nut or bolt, are not tightened simultaneously. Tightening one bolt in the group has an effect on the preload in other previously tightened bolts in the group. Such effect is called elastic interaction, or sometimes bolt crosstalk.
Yes
Using improved angle control method with self-alignment, Delnut can be preloaded according to incorporated scale. This method is far more accurate than a torque wrench and constitutes an inbuilt feature of the fastener.
Nuts are torqued with an ordinary spanner. No specialised tools are needed.
An additional feature has been under development, which will allow accurate preloading by feel of a sudden change in torque, whch will be useful in combination with power tools: electric, pneumatic or hydraulic. Both methods are very accurate.
Clamp force maintenance
The residual tension in an assembly drops significantly straight after preloading and further deteriorates at a slow but steady rate. Initial preload loss can vary between about 2% and 10% of the actual preload level in the bolt.
Loss of tension is primarily attributable to the joint relaxation, when elements under stress, such as threads, washers, seals or gaskets initially resist clamping pressure, but over a period of time, yield plastically into permanent deformation.
It is well understood that proper preloading of fasteners is vitally important, so must be mechanism to prevent its loss.
Another source of preload loss could be self-unscrewing due to the build-up of residual torsional stress during preloading.
No
There is no clamping force loss prevention feature incorporated in prevailing torque fasteners.
No
Serrated fasteners damage the work-piece's surface during preloading and subsequently embed themselves into it. Both processes cause deterioration of the joint and a loss of preload.
No
For proper operation wedge-washers must be fixed to the threaded fastener and the work-piece. This is achieved by the use of some sort of ridges or protrusions forged on the opposite side of the washer from the wedges, which embed themselves into parts of the joint they are in contact with. In other words, they cause creep of the joint by design. The loss of preload starts immediately after assembly.
No
Loss of preload is proportional to the initial tension in the assembly. Achieving proper preload does not help the subsequent loss.
No
Stress relaxation is a major problem with the multi-jackbolt fasteners. This is because of high stresses concentration in the jackbolts, which creep on the thread and at the interface with the main fastener.
Yes
Unlike any other fastener, Delnut holds the residual load at a level close to the preload. This is because Delnut is elastic throughout;
- It has an elastic nut and a resiliently deformable washer and
- It permanently self-tightens in a vibrating environment.
Temp. effect correction
Increase in temperature causes reduction in bolt's modulus of elasticity, effecting proportional loss of tension in the joint. If all of the materials of the joint and the bolt are the same then any changes in temperature will reduce the joint loadings. However differential thermal expansion may compensate for that loss. If the joint materials have coefficients of thermal expansion different to the bolt material, changes in the joint loading result from changes in temperature. As the joint is heated, the clamped parts expand causing a temporary increase in tension. An aluminum joint expands significantly, causing substantial increase in the tension of the bolts. As the joint cools, the clamping material retracts causing the joint to return to its original state. Increased tension may cause material flow under the bearing surface, which on cooling will cause drop in joint tension below the original value.
Elevated temperature may also cause creep, which is plastic or permanent joint thickness change due to stress and temperature that occur below the yield point.
When the joint (for example a wheel/fastener assembly) is heated, the wheel coating tends to act as a lubricant between the wheel surface and the washer. Performing a torque audit at that point produces torque values considerably lower than the installation values.
No
Prevailing torque are rigid fasteners, which are affected by temperature changes.
No
Serrated fasteners do not cope with temperature changes any better than ordinary bolts and nuts.
No
No built in temperatures effect compensation feature.
No
Just as with conventional fasteners, there is no temperatures effect compensation feature in tension sensors.
No
There is no special geometry, which would help with the effect of changing temperatures in multi-jackbolt fasteners. For high temperatures applications jackbolts are made from heat resistant seels.
Yes
The special retainer washer is rigid enough to hold elastically massive high tensile nut's proof load. It doubles joint elasticity preventing formation of creep, so it keeps holding set preload forever, while compensating for temperature changes.
Wedges prevent unscrewing by wedging the thread, so unlike friction, they constitute positive mechanical barrier. Wedge-fasteners have been patented over a century ago, but only recently have become more popular. When intelligently incorporated into a fastener, wedges are capable of greatly improving anti-vibration fastening.