Zero Breakdowns Strategies – Step 4 – Improving Design Weaknesses
Design weaknesses can be improved in the equipment by strengthening the various parts to extend component life. This may take the form of some type of wear resistance, where a material is changed in a high wear area to a material that has a higher wear rating than the components around it.
Corrosion resistance may require the changing of material that is more corrosion resistant than the material around it to improve the process reliability. There may be occasions where stress in the design of the components is the issue and the design must be changed to minimize the existing stress and fatigue.
It may also be necessary to change materials and shapes of items so that they increase their reliability there may also be the need to improve assembly accuracy so equipment is assembled correctly. While all of these are great ideas to improve design weaknesses, there is one major problem with assuming a design weakness. How many people really know the true design life of basic components?
For example, what is the design life of a V-belt. In some companies V-belts are changed every three to six months, and this is a waste of manpower, spare parts, and equipment capacity. The true design life of a properly rated, properly installed, and properly maintained V-belt is three years of continuous operation or 24,000 hours. Yet, many companies change V-belts much more frequently.
However, this is not a design problem; it is usually an installation and maintenance problem. For example, how many companies really follow the proper design procedure outlined by the manufacturer when installing a V-belt? Usually it is a small minority. Installers may pry the belts on, run belts on, jog units to get belts on, but they do not follow proper procedures. They seldom check alignment, they seldom the check tension properly, or they seldom check for sheave wear. All of these can be root causes of major reductions in the belt life.
Another example is roller chain. The design life of roller chain properly installed properly rated and properly maintained is seven years. Roller chain installed improperly and not properly lubricated has an expected life of nine days. This is a tremendous difference in life expectancies. It is not a design problem, but more likely it is going to be a maintenance and installation problem. For example, some companies will continue to install a new roller chain over worn out sprockets. A chain and sprocket should be changed at the same time (a maximum of 3 replacement chains can be achieved). While this may seem excessive, it is the recommendation. (Just ask any motorcycle owner)
However, some companies may find that they can allow one sprocket to wear out 4 or 5 chains before changing the sprocket. In reality, it must be kept in mind, that when the chain is worn out, a similar amount of hardened material is worn from the sprocket. The tooth geometries changed as the chain the wears against them. Once the chain has worn, the tooth geometries are changed enough that it will increase the wear on the second chain, a corresponding increase will occur for the third chain, and the fourth chain, accelerating the wear until the chain fails very shortly after installation. It is only by understanding proper installation and maintenance practices around these components that design weaknesses can ever truly be identified.
Bearings are another example. How many bearings in a typical plant actually achieve the L-10 rating of the bearing? In most cases the bearings never achieve the design life, simply because they are mishandled installed incorrectly, or maintained incorrectly. One study showed that less than 5% of pump bearings in the petrochemical industry ever reach the L-10 rating. The actual rating is over 15 years, yet the majority of the bearings (95%) average just over a year of actual usage. Some companies changed bearings on a weekly or monthly basis and, when in reality they should be achieving years of use from the bearing.
Another study showed that just about 2/3rds of bearing failures are caused by user induced problems. These problems would include maintenance issues, operational issues, and construction/installation issues.
Best or Worst Practices?
In examining this ZBS step, it must be asked what atrocities do most employees commit against the basic components that prevent achieving design life?
For example, do you see plant technicians installing bearings with hammers? What impact does this have on the design life of the bearing?
Have technicians ever been observed welding on the same plane with bearings allowing the electric arc to pass through the bearing? This again, dramatically shortens the life of the bearing.
In the case of roller chain, repair sections are placed in chain or special links are put in chain. This introduces different forces in the chain drive that accelerate the wear. The chain will experience tight loads and light loads as the worn and new chain simultaneously operate. This creates tremendous wear on all affected components.
Summary
All of these issues must be considered before assuming a particular problem is a design problem with all components. In many cases companies will blame chronic equipment problems on the design engineer or the equipment manufacturer. Upon closer examination, it is found that in most cases, the root cause of the problem is a
maintenance or operational issue. These issues should be addressed first, then the true design error will be clearly identified. Then the design problems can be dealt with properly.
However, as a final note, do not assume that chronic equipment problems are always design issues. Most equipment problems are related to a basic root cause already mentioned in these blogs. If these issues are examined first, the solutions can be quickly implemented. This process will then make available resources to concentrate on solving what are really design problems.[/fusion_text]