Weak link

A good example is the impact just-in-time supply chains have on a production system. Automotive assembly lines can grind to a halt if even a minor supplier has a production interruption. The situation is not confined to just-in-time situations. It s not uncommon for top-level operations managers to patrol their storerooms with shortage lists in hand — even with several months inventory in the warehouse They are the victims of weak links either in their supplier base, their ability to stay in touch with that supplier base, or in their own abilities to understand and cover their needs. 

Such failures from weak links won t always announce themselves in dramatic fashion so they are immediately obvious. Small changes like late deliveries and stock outs may signal a slowly evolving breakdown somewhere. What is only irritating today can lead to greater problems tomorrow. Without preventive measmes, all in the chain will suffer. 

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Laminates ate a special form of composite material or reinforced plastic because the continuous reinforcing ply of fibrous material imparts significant strength in the x—j plane. The strength along the axis results from interlaminar bonding of resins. Very few fibers ate oriented in the direction, so it tends to be the weak link in this type of composite. 

The major assumption in weak link design is that the eost of failure of the maehine that is to be proteeted from an overload situation in serviee is mueh greater than the eost of failure of a weak link plaeed in the system whieh is designed to fail first. The situation is primarily driven by various eosts whieh must be balaneed to avoid at one extreme the eost of failure of the system, and the other overdesign of the elements in the system. The eost faetors involved are typieally ... 

Cost of failure with the weak link relative to the eost of failure without the weak link... 

Cost due to maehine downtime if the weak link fails prematurely... 

Cost of inereasing strength of maehine elements to aeeommodate the weak link. 

However, too great a separation, and overdesign may oeeur. The overload eondition is represented by a unique stress, whieh is very mueh greater than the working stress, applied suddenly whieh eauses only the weak link to failure due to stress rupture. 

Experimental determination of ultimate tensile strength of the weak link material... 

We know that the eoeffieient of variation, C, of the applied torque is approximately 0.1, and that the final loading stress variable will have a similar level of variation beeause the dimensional variables have a very small varianee eontribution in eompar-ison. We also know the ultimate shear strength parameters of the weak link material, therefore substituting in equation 4.91 and rearranging to set the right-hand side to zero gives ... 

The torque eapaeity of the pump shaft must be greater than the torque eapaeity of the shear pin in all eases. We assume that failure of the pump shaft oeeurs at the interferenee of these two torque distributions. From equation 4.89, the torque eapaeity of the shear pin ean be determined by substituting the ultimate shear strength of the weak link material, for L, giving ... 

Figure 4.61 Weak link torque capacity shown relative to the applied torque and the torque capacity of the pump shaft...

As a guide, SM should be less than 10 for all eases of failure severity to avoid over-design. The distributions of the applied torque, weak link torque eapaeity and pump torque eapaeity are plotted to seale in Figure 4.61 for eomparison. 

The press had been designed with a capacity to deliver 280 kN press force and to work at a production rate of 40 lids per minute. Calculations to determine the distribution of forming loads required indicated that the press capacity was adequate to form the family of steel lids to be produced on the machine. One of the major areas of interest in the design was the con-rod and pin (see Figure 4.66). The first option considered was based on a previous design where the con-rod was manufactured from cast iron with phosphor bronze bearings at the big and small ends. However, weaknesses in this approach necessitated the consideration of other options. The case study presents the analysis of the pin and con-rod using simple probabilistic techniques in an attempt to provide in-service reliable press operation. The way a weak link was introduced to ensure ease of maintenance and repair in the event... 

Selection and insteillation of an integrated air pollution control system do not end the concern of the utility industry. Maintenance and operational problems of the system are considered by many engineers to be the weak link in the chain of power generation equipment (6). The reliability of the... 

Polymer reactivity differs from the reactivity of simple molecules in two special respects. The first of these is due to the fact that a number of weak links... 

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

Weak links, particularly terminal weak links, can be the site of initiation of a chain unzipping reaction. A monomer or other simple molecule may be abstracted from the end of the chain in such a way that the new chain end is also unstable. The reaction repeats itself and the polymer depolymerises or otherwise degrades. This phenomenon occurs to a serious extent with polyacetals, polyfmethyl methacrylate) and, it is believed, with PVC. 

By preventing the initial formation of weak links. These will involve, amongst other things, the use of rigorously purified monomer. 

By deactivating the active weak link. For example, commercial polyacetal (polyformaldehyde) resins have their chain ends capped by a stable grouping. (This will, however, be of little use where the initiation of chain degradation is not at the terminal group.)... 

As previously mentioned, some urethanes can biodegrade easily by hydrolysis, while others are very resistant to hydrolysis. The purpose of this section is to provide some guidelines to aid the scientist in designing the desired hydrolytic stability of the urethane adhesive. For hydrolysis of a urethane to occur, water must diffuse into the bulk polymer, followed by hydrolysis of the weak link within the urethane adhesive. The two most common sites of attack are the urethane soft segment (polyol) and/or the urethane linkages. Urethanes made from PPG polyols, PTMEG, and poly(butadiene) polyols all have a backbone inherently resistant to hydrolysis. They are usually the first choice for adhesives that will be exposed to moisture. Polyester polyols and polycarbonates may be prone to hydrolytic attack, but this problem can be controlled to some degree by the proper choice of polyol. 

The JHA allows you to identify weak links in the system. Once you discover the weak links, you can update the JHA to reflect the needed changes. 

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