Threaded Fastener Torque Tightening Program
The Problem
A survey of car service managers completed
in the United States, indicated that 23% of all service problems
were traceable to loose fasteners. Such problems are common
across many industries. A major cause of fastener loosening
is inadequate tightening. A single bolt, inaccurately or incorrectly
tightened, can lead to the failure of the complete product.
Torque Controlled Tightening
The most prevalent controlled method of tightening threaded fasteners is by tightening so that a specified torque is achieved. This method is generally known as torque control. The major problem related to this method is that the fastener clamp force (referred to as preload) generated as the result of an applied torque is dependent upon fastener design and the prevailing frictional conditions. Despite these problems, it is still the most popular way of ensuring that an assembled bolt complies with an engineering specification.
The Importance of Tightening Torque
Most Engineers already know the importance of a high preload in maintaining joint integrity. For torque controlled tightening, achieving the right preload is dependent upon the correct tightening torque being specified. Without the proper analytical tools and information, specifying the correct torque can be problematical. Whether you are a Service or a Design Engineer, you frequently need to know what is the correct tightening torque. For many types of threaded fasteners, this information is either not available, or not readily available.
Analysis of Bolted Joints
A great deal of sophistication can nowadays be applied to the analysis of structures. Frequently however, the assumptions made in the analysis regarding the scatter in a bolts preload, is either simplistic or is ignored. That is, it is often assumed that a bolts preload will not vary from a single calculated or test determined value. Frequently, this can be a fatal flaw in the structural analysis. Fluctuation in a bolts preload is mainly caused by variations in the frictional conditions and the method of tightening being used. If this fluctuation is outside the assumed value, product reliability problems can result. Such reliability problems can manifest themselves as fluid leakage past a gasket, fatigue failure of the bolt itself or of the whole joint. Being able to predict what will be the minimum fastener preload can be crucial to the reliability of a product.
The TORQUE Program
TORQUE is a program which is designed to assist the Engineer in the solution of problems related to the torque tightening of threaded fasteners. The program determines both tensile stress due to the elongation of the fastener, and the torsional stress due to the applied torque. It accounts for the frictional effects in the thread and between the nut face and clamped surface. Account can also be made for the effects of a reduced shank diameter (smaller than the thread size) and a prevailing torque. The prevailing torque is the torque required to run a nut down a thread before engagement with the joint surface. This resistance can be provided by a nylon insert or a distorted thread profile. A prevailing torque can significantly affect the amount of preload provided by the fastener for a given tightening torque.
Versatile Analysis Options>
The
program analysis options include the determination of :
a) The
tightening torque and preload so that a given stress level is achieved
in the fastener.
b) The
tightening torque having specified the preload required.
c) The preload having specified the tightening torque.
Menu Options
An in-built database is provided containing
information appertaining to thread pitch, root, effective
and nut bearing diameters for standard fasteners. The default
metric coarse thread data used by the program can be changed
to metric fine, Unified Coarse or Unified Fine by selecting
the appropriate menu option. Similarly, the default metric
unit of measure can be changed to various imperial units by
selecting an appropriate menu option.
Help
TORQUE runs
under Windows operating system. TORQUE is provided
as a download from our web site.
Presented below is further information related to this program.
Example
calculations (METRIC) completed by TORQUE
Example
calculations (Inch/Lb) completed by TORQUE
