Focus on Alignment: Dancing with a soft foot
Alignment of machinery is critical in plant operations, and soft foot conditions are especially troublesome. Here's how various alignment systems can help.You would probably think it foolish to have w...
February 1, 2000 | By John Lambert
Alignment of machinery is critical in plant operations, and soft foot conditions are especially troublesome. Here’s how various alignment systems can help.
You would probably think it foolish to have work done on the front end of your car and not have the alignment checked and if necessary adjusted. If the front end is out of alignment the car would start to vibrate, sometimes excessively, right up the steering column into your hands. This would undoubtedly put greater stress on the front suspension and steering, causing expensive damage. You will see this most of all in the tires. You can go through a set of tires pretty quickly when the front end is out of alignment. In addition, your fuel usage will increase because of the extra power required to push that misaligned car around.
You would not knowingly leave your car in a misaligned condition because you know what the consequences will be. Yet machinery is installed and left in a misaligned condition on a regular basis throughout all of North America’s industries.
When most maintenance personnel think about misalignment they think of shaft-to-shaft alignment such as an arrangement with two separate shafts and a flexible coupling. But it’s much more than that. I see misalignment in chains and sprockets, V-belts, pulleys, cylinders, and shafts of coupled driven machines. The reasons for this misalignment are: a lack of knowledge, a lack of tools (instruments), and in some cases a perceived lack of time in which to do the job. However, the cost of all this misalignment is in the billions of dollars.
For now, let us concentrate on shaft-to-shaft misalignment. To get a better understanding of the problem, visualize how many pieces of coupled driven machine units (pumps, compressors, etc.) there are in the industrial area that you work in. The number would be quite high. Can you imagine how many there are in a huge industrial area like the Great Lakes Region of North America, for example?
Now consider the following: The SKF Bearing Maintenance Handbook states “Investigations made in the U.S.A. have shown that misalignment can be traced as the cause of 50% of the breakdown in rotating machinery,” and “A 20% load increase from misalignment reduces the calculated bearing life by almost 50%.”
The initial installation of a piece of equipment has to be done right. The alignment of the shafts (or their misalignment) will have a big influence on the life of the machine. Strangely enough, if the machine goes down after 10 months of operation for a motor bearing or pump seal failure, the alignment is seldom questioned. It is usually equated to a bad bearing, or a poorly installed seal.
Maintenance people often say that couplings can handle a lot of misalignment, or that since a machine runs very slowly, they need not be concerned with alignment. I don’t buy that argument! Even with the most forgiving coupling, the constant flexing back and forth of a misaligned shaft causes forces to be transmitted back through the shafts and bearings. These forces can drastically reduce the life expectancy of shafts, bearings and couplings. Coupling manufacturers generally do not try to mislead you about the capability of their couplings. They simply point out that one of the benefits of their couplings is that they have a high tolerance to misalignment. However, to them, shafts, bearings and seals are someone else’s problem.
The soft foot problem
There is more involved in effective machinery maintenance than just shaft misalignment. Soft foot is also a major problem that results in premature machinery failure. Soft foot is a condition that occurs when the mounting feet on the machine do not lie in the same plane as the base to which they are bolted. Soft foot not only makes it impossible to align the shaft, but also causes misalignment internally within the machine.
This problem occurs not only in coupled driven machines, but can also be found in many other pieces of equipment, such as chain and belt driven machine units, air motors, cylinders, gearboxes, etc. In a discussion I had with an engineer from Timken Bearings, he said they would like to see internal misalignment down to 0.001 in., and he wasn’t joking. You can spend a lot of time finding and correcting soft foot, but it can be frustrating because you can create soft foot just as easily as you can correct it. However, the effort to correct it is time well spent. There is much more that can be discussed on this subject, but for now let us say that the problem mainly is one of poor machinery installation practices.
To demonstrate some of the poor installation practices, let us consider the installation of a belt driven machine unit. The machines are moved into position and the sheaves are installed loosely on the shafts. The mechanic roughly aligns the sheaves and then tightens them on to the taper lock bushings. To align the sheaves, a straight edge, or quite commonly a piece of string, is used to align the four edges of the sheaves. Then the belts are installed and adjusted to the correct tension. After a quick re-check the job is considered finished.
Now consider this. The frame that this machine unit sits on is probably bolted to the floor, and because the floor probably is not level, the frame will take on the same contour as the floor. If you bolt the machine to a twisted frame you will distort the machine’s casing as well. In most cases the bearings which support the shaft inside the machine have extremely close clearances, and the distortion in the casing can result in the reduction or removal of these clearances. Therefore, a soft foot correction should be done.
Another common problem can occur when tightening the taper lock. If you tighten one side slightly more then the other, it will skew the sheave. This causes the drive to shimmy when rotated and to vibrate axially along the shaft, hammering at the supporting bearings. The belts may be able to withstand these forces, but what about the rest of the equipment? To avoid this problem, a simple dial indicator should be used to check the run-out. A complete discussion of the proper alignment and belt tension is best left for another article, but note that there are low-cost instruments on the market today that will do a much better job than the string or straight edge–and do it faster.
One of the constant themes in vogue these days is “doing it right the first time.” Rework, repair, or replacement is very expensive, not only in dollars but in time, especially lost production time. What you need, to do the job right, is the right attitude. You need to be committed to the belief that “I will get the full life expectancy from all of our equipment.” Once you are committed to this belief, the majority of the instruments, tools and training you choose will be aimed at prevention. The primary focus will no longer be predicting the time of failure so that you can do a breakdown analysis. Your goal will be to prevent failure.
At a minimum standard, you should have the knowledge (training) of complete installation procedures, including shaft-to-shaft alignment. Even if you choose to have someone else do the work, such as a hired contractor, either you or someone on your staff should be able to verify that the work has been done correctly the first time.
Defining your requirements and goals for your machinery installation should be your starting point. Once you know what it is that you want to achieve, you can buy the right tools or training for the job. For instance, there is not much point in buying a dial set that will only allow you to do rim and face alignment, when for many pieces of equipment the reverse dial procedure is the one that is recommended. You may want to consider a laser alignment system, and if so you will want to make certain that you choose the system which best suits your requirements (see Guidelines for Choosing a Laser Alignment System, p. 22).
Budgeting for alignment
What portion of a plant maintenance budget should be spent on aligning equipment? You should add up the total cost of all the machinery in your plant that
you should be aligning. If you then calculate 1% of that figure, the resulting number is about what you should be spending annually on alignment instruments and training.
This means smaller plants, with say 10 pumping systems, require a dial system for doing reverse dial alignments. A medium-sized chemical plant should have a basic laser system for shaft-to-shaft alignment. A larger plant with gear drives, steam turbines, jack shaft and similar equipment will need a mid-range laser. Larger firms in heavy industries such as shipping, steel mills, paper mills or mines should be using a geometric system in combination with a good laser shaft alignment system with long-range capabilities.
Justifying the cost
Management has to see an improved bottom line to justify the cost of these instruments. The simplest way is by adding up all the unscheduled downtime, including the cost of the lost production, spoiled product, the overall breakdown repair cost, the salaries paid idle production workers, and the premium salaries paid to maintenance workers in the form of overtime, etc.
Another justification can be the reduction in energy costs on equipment that has been properly aligned. Still another way to justify the purchase of an instrument is by reducing the cost of the secondary damage done to the equipment during the failure. Failure due to misalignment quite often includes damage to the other components in the system. It is not necessarily the same failure as that seen when a component wears out gradually from normal wear and tear. Quite often a component failing due to misalignment goes out with a bang.
It’s one thing to have to replace the bearings on a motor. It’s a lot more serious to have to send that motor out for repairs due to it being damaged. If a bearing seizes on the shaft of a gearbox, it can ruin the complete unit. The outer race on a seized bearing can spin in the housing of a pump or a split pillow block, causing the replacement of not only the shaft but also the housing. The additional costs for such repairs are huge. Now if you add on all of the other costs incurred due to the failure, it can prove to be a very expensive.
Doing the justification is the hard part. Although the front-line supervisor knows the value in having the right tools to do the work, he is also one of the busiest people in the plant. He has to manage up as well as down, so he may not have the time to figure out the justification. Quite often this is where the alignment decision ends.
Sometimes the decision is passed along to the engineering group. This group, when given an opportunity to buy something new, quite naturally will look for something that it can relate to. But that may not be the same thing that is needed in the maintenance shop. It is important that the end users of the instrument be involved in its selection. Although these are sophisticated instruments, the end user has to view it as a simple tool he can pull out of the box and use. He may not be using it on a regular basis, but he should not have to sit down and read the operation manual every time he needs it.
Another interesting piece of information comes from the Goulds Pumps division of ITT Industries. It reports that “Over the lifetime of a chemical pump, only 5% of the total cost is in the initial purchase price; 95% of the cost is installation, operation and maintenance of that pump.” If you can guarantee that the installation of a pump is done properly, you can reduce the operational cost by decreasing the amount of energy consumption. Also, because of a reduced amount of vibration due to the improved installation, the maintenance cost will be greatly reduced. This is a very pro-active form of preventive maintenance.MRO
Would you like to comment on this article? Contact John Lambert of Benchmark Maintenance Services Inc., Toronto, Ont., by e-mail at email@example.com. For more information, visit the web site at www.easy-alignment.com. You’ll also find additional alignment information, including a case study, at www.mromagazine.com.
Guidelines for choosing a laser alignment system
When purchasing a laser system you have to be very careful. If your requirements are simply for shaft-to-shaft alignments with a distance of up to 20 or 30 ft, you can buy a system for about $7,000 or $8,000 that will more than meet your needs. Make sure that it is a robust system that will withstand the industrial environment, and that it is user friendly. You will want rugged, dependable equipment that is easy to use and understand.
If you want something with a little more capability, such as programs for straightness checks and vertical alignments with an internal memory for storing readings, and the ability to communicate with your PC, expect to pay somewhere in the $13,000 range. If you need something for longer spans of up to about 65 ft, in a system that can grow and be added to as your awareness of proper installation requirements grows, anticipate spending in the range of $15,000 or more.
The main thing to remember is to not get caught up in the hype that surrounds these instruments. In some cases there can be a lot of smoke and mirrors. Some systems will allow you to believe that they can do a lot more than they really can and all you have to do is push the buttons. Don’t believe it!
For example, the soft foot programs they use may only measure shaft deflection, but the shaft may not always deflect when you have soft foot, so to be certain, you will still have to measure it yourself using feeler gauges. You cannot blindly process numbers through these display units and expect magical solutions–it doesn’t happen. You still have to do certain manual checks and apply your grey matter in the installation process. I highly recommend that you keep this process as simple as you can. I have seen laser systems that were bought with all the bells and whistles on them and not used because they were too complex.
You may want more out of a laser system. If you believe that prevention is better than cure, you will want to invest in one with geometric capabilities. Geometric systems take advantage of the speed and accuracy of laser technology and can measure straightness, squareness, flatness, perpendicularity, parallelity, level and plump. They start at about $25,000 and can be used in a variety of applications in your plant, such as making rolls parallel on a printing press, or in a paper mill. Also, you can use a squareness program for setting up CNC machines. You can use a spindle laser and program for measuring spindles in tooling or drilling machines, and use the centre of circle program for measuring the straightness of bearing journals or extruder tubes. There are many applications for these systems. Quite often the limiting factor is only one’s imagination.