垫片百科
-
Clamp Load: Establishing and Maintaining
By Larry Pyle, President, LFPtechnologies
Every gasketed joint is unique. Often, the gasket is the compromising component that has to make up for the deficiencies of the rest of the system. It has to perform its’
principal function of establishing and maintaining a seal whether it be gaseous or fluid. The principle failure of a joint results from the loss of the tension or the initial load
imparted on the assembly by the fasteners. The design objective is to establish an effective and durable seal and to maintain it throughout its service life. It used to be that the gasket was the last consideration given when the joint was designed. Quite often, flange stiffness was weak and yielding, bolt patterns weren’t symmetrical, fasteners were short with little stretch, or insufficient in providing enough clamping load across the entire gasket. These conditions required unique solutions to the gasket design which often required additives or enhancements to a simple gasket.
The success of any gasket is highly dependent on the other components that make up the joint. The key is the fastening system, and its’ ability to establish adequate and predictable compressive loads on the gasket is probably the most important factor. In its’ simplest form, it takes load to seal. Not only does the joint have to provide sufficient initial load at installation but also must maintain sufficient load
throughout the life cycle of the gasketed joint. There are many factors that go into the equation.ESTABLISHING INITIAL CLAMP LOAD
What makes the gasket seal is to establish a minimum recommended compressive stress on the material. Clamp load in excess of this minimum is desirable if available through the fastener system. This stress compresses the
gasket material, allowing it to conform to the surface finish and flatness conditions of the flanges. During this compression process, the inherent porosity of most gasket materials is reduced to impede the minute flow of gases or liquids being sealed. The source of this compressive load isthe fastening system, bolts or screws in most cases. This load is a linear function related to the stretch of the bolt or screw.
Friction:It is the tension in the fastener that is the important factor,not the to rque used to produce it. Torque is simply related to tension by the following equation T = μFD (T = torque (lb-ft), F = force (lbs), D = fastener diameter in inches,μ= coefficient of friction). The big IF in this equation is the coefficient of friction. It is not very reliable since so many factors can change the relationship between Torque and Tension (some sources say that there are some 30 or 40 factors).
Alternative tension control:
As an alternative to friction control, manufacturers have attempted to eliminate friction as a factor altogether by going to other tension control systems. Torque-to-yield
and Torque-angle techniques are commonly utilized. These are most commonly used in automated assembly operations that are performed with sophisticated machines.
Torque to Yield Method:
Since friction is the largest variable that affects the torque tension relationship, alternative methods were developed to ensure that the effect of friction was minimized and that
tension in the fasteners are predictable and consistent. It was determined that the metallurgy of the fasteners can be tightly controlled so that they yield, or permanently deform as very precise elongations with very predictable force. Thus,the Torque-to-Yield method has been utilized extensively,particularly by OEMs. Sophisticated equipment has been developed to apply this technique. The graph on the next page illustrates a typical result,a tight control of clamp load and elongation (stretch).
Torque –Turn Method or Torque-Angle Method:
Another method that uses a similar principal is called the Torque-Turn method. Again, it assumes that the bolt properties are carefully controlled by metallurgy. The method consists of applying a fractional portion of the final torque to each fastener and then rotating the application tool through a fixed angle or angles. This method is also
useful in detecting a lot of production related problems:blind holes, incomplete tapped holes; crossed threads, soft parts, chips, etc. Measuring both torque and turn makes it
possible to spot problems of this sort. In one technique, an electronically controlled air tool will produce first a preset torque, then a preset turn of the fastener. The control system will examine the final torque required to produce the final turn. If everything is correct, the torque and turn values will fall somewhere within an acceptable “window” as illustrated in the following figure. The Torque-Angle technique is often used in field conditions where consistent tension is required and is done manually using torque wrenches.
Joint Flanges:
This comprises the metal structure on either side of the gasket. They can also be made from engineered plastic.Structural stiffness is another key to the gasket’s success.
This stiffness is a function of:
• Flange thickness
• Reinforcing ribs or other third dimensional attribute
• Modulus
• Fastener spacing
• Dissimilar metals can create gasket scrubbing due to
thermal cycling. This can be destructive.Gaskets:
Gasket materials are engineered to reduce the effects of their environment on their load bearing properties. All gasket materials have varying degrees of the following properties:
• Fluid resistance/compatibility
• Creep relaxation properties
• Compressibility/recovery
• Tensile strength
Selecting the right combination of these material attributes to match the requirements of the application is the critical step.RETAINING INITIAL CLAMP LOADS:
The objective of any gasket is to maintain as much of the initial assembly load as possible. This is rarely possible but can be controlled to acceptable limits.
The largest contributors to the degradation of the clamping forces are:
• Chemical attack of the gasket materials will lead to rapid degradation of essential gasket properties including load bearing.
• Creep-Relaxation. Heat and vibration are significant catalysts in that process.
Remember that load is established by stretching the fastener and load loss is as a result of stretch loss. This loss is called Creep-Relaxation or sometimes Stress-Relaxation,
depending on the industry.The formula for this is:
Creep-Relaxation (%) = Tension (load) loss (%) =100 ( △Linitial - △Lfinal) / △Linitial
△Linitial = initial fastener stretch
△Lfinal = residual fastener stretchIn general these statements apply:
• The thickness of the gasket will contribute to creep (the thicker the gasket the more creep).
• Stress relaxation as a percentage of the initial load is greatly influenced by the length of the fasteners.
• Fasteners have elongations that are directly proportional to their relative lengths.
• Smaller diameter fasteners stretch more than those with larger diameters for the same length.
• Some creep will occur in a joint even in the absence of a gasket.Gaskets are not the sole contributor to creep relaxation.Temperature and chemical attack are catalysts to creep.Vibration is another catalyst that can increase creep and degrade gasket performance. Internal pressures can rupture a gasket often with messy and sometimes catastrophic consequences。
Depending on the complexity of the sealing environment,additional features are often required when a simple, cleancut,gasket doesn’t do the job. These are the subjects of
other articles.上一篇文章：如何判断垫片品质的优劣？ 下一篇文章：无石棉垫片材料制造工艺介绍 更新时间：2016.02.01 查看次数：0