Pruftechnik Blog

Shaft Alignment

How to diagnose and correct soft foot (part 2)

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Eliminating soft foot before attempting machinery alignment will save you time and money

In the first installment of this two-part blog series about soft foot, you learned how to identify the symptoms, effects, and types of soft foot conditions. In this second post, you’ll learn how to diagnose and correct the condition and what types of tools to use.

It’s estimated that two-thirds of all rotating machinery have soft foot conditions. Soft foot is also the root cause of most alignment issues and is considered to be the most critical thing to check before performing an alignment. So why is soft foot one of the most overlooked machinery problems?

Granted, the process of diagnosing and correcting soft foot can be frustrating, and it often takes longer to fix than the actual machine alignment. However, if you can remain calm and think critically, the effort always has a huge payoff which is preventing machine destruction.

Soft foot toolkit (Diagnosing soft foot)

Several ways exist to diagnose and measure soft foot. It’s essential to apply the following methods during every machine alignment and installation to determine if there’s a soft foot condition.

Having the right devices on hand can make detecting soft foot go smoother and faster. Here’s a list of tools to help you measure for soft foot and eliminate any debris found under and around the feet to ensure machine alignment.

  • Image 1 – Feeler gauge
    Image 1 – Feeler gauge

    Feeler gauge: A feeler gauge is used to measure gap widths or the clearance between two parts. The tool has several steel blades with different thicknesses that can measure as little as 1/1000 of an inch or up to a quarter (¼) inch.

 

  • Micrometer or digital caliper: A micrometer can also measure gap thickness. The tool is used to verify shim thickness. A caliper is used to measure shim stacks before inserting them under the feet. This verifies that the shims are the thickness as labelled and not defective.

    Image 2 – Digital caliper
    Image 2 – Digital caliper
  • Wire brush: This clears away the debris or corrosion you may come across while checking machinery feet.
  • Cleaner/degreaser: Use a heavy-duty degreaser in combination with the wire brush to clean existing shims and contact surfaces during the soft foot correction. Brake cleaner works well, but you might also have success with other well-known commercial cleaners.

 

  • Laser shaft alignment system: A laser system can be helpful when measuring for soft foot and is easy to use. This sophisticated tool walks you through the measurement process via a series of screens and includes an auto diagnosis. Although you can use either a dual or single laser shaft alignment system, a dual system can be more challenging to use.

 

Correcting soft foot with shims

Using shims to correct machine foot issues is usually very effective. However, if the frame or mount is bent, twisted, or cracked, or other overall machinery destruction has occurred, correcting the condition can be quite tricky.

In most cases, the machine you’ll be working on will already be shimmed. It’s a good practice to clean any shims that are not new and also clean the base plates and feet to eliminate any dirt and debris. If you do not remove the corrosion, paint, and grime that can accumulate over time, it’s possible to form a new condition, squishy foot, during alignment.

5 rules to avoid squishy foot

Here are some good rules of thumb to follow to better ensure that you don’t create a squishy foot condition while trying to eliminate another soft foot condition.

  1. Use shims big enough to cover at least 80% of the footprint. Shims come in many sizes, and if the machinery requires large shims, they’re usually fabricated on site.
  2. ShimApply no more than four shims under each foot. If you add too many shims, you risk producing a spring-like situation, i.e., squishy foot.
  3. Sandwich thin shims in between thick shims. It makes placement easier and eliminates the potential for shim creasing.
  4. Measure stacks with a micrometer to verify shim thickness. Use it to make sure things add up as they should because errors or defects sometimes happen during manufacturing.
    Shim rules pic
  5. Measure shims over 0.030 to verify thickness. Some sources might recommend lower or higher thicknesses, but ultimately, it’s your decision.

By following these soft foot best practices, including applying the right methods and tools to accurately diagnose and correct the condition, you set yourself up for success. Always remember to check for soft foot before aligning shafts. It will save you time, money, and headaches.

For a more in-depth look at soft foot, watch the Fluke Accelix Best Practice Webinar, “A hard look at soft foot: detecting machine frame distortion before it causes major issues.”

What to know about soft foot: symptoms, effects, and types (part 1)

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Soft foot can damage or destroy machinery

Soft foot is a commonly used industry term describing machine frame distortion. It occurs when rotating machinery is set on its base, frame, or sole plate and one or more of its “feet” fails to make contact with all of the “foot points” on the frame.

Chair and machine feetAn easy way to visualize soft foot is to imagine a four-legged wooden chair that wobbles because one leg is shorter than the others. Similarly, if all of the foot points on a machine base aren’t entirely even, the machine will wobble. To correct the condition, you must adjust all of the foot points and the entire machine frame to achieve precise alignment.

Misalignment is the most significant and damaging consequence of soft foot. This condition can also lead to seal failure, distortion, misalignment of bearings, increased load on bearings, and a bent shaft. Together or separately, any one of these situations can destroy machinery if left unchecked.  

Why is it important to understand the effects of soft foot on rotating machinery? 

Symptom Effect 
The centerline of rotation shifts Destroys the machine from the inside out 
A change or shift in shaft position occurs Warps the machine
Residual vibration  Loosens feet over time, causing shims to work their way out
Repetitive vibration caused by loose feet Creates fretting corrosion and damage to the machine base
Cyclical fatigue due to areas experiencing highly concentrated stress Produces cracks that spread along the machine housing

You must identify what’s causing the soft foot and correct the issue. Finding the problem can be frustrating. But with knowledge, patience, and the right tools, you can diagnose the cause of the condition and correct it. 

There are four types of soft foot: parallel soft foot, angular soft foot, squishy foot, and external force soft foot. Here’s an explanation of each condition. 

4 types of soft foot 

parallel soft foot
Figure 1. Parallel soft foot

1. Parallel soft foot is usually the easiest to detect and the most common type of soft foot. It happens when a foot doesn’t reach the base and creates a gap between the foot and the frame. As the gap begins to close with each tightening of the bolts, the machine frame starts to bend to fit the baseplate, causing misalignment. You can usually correct the problem by using shims — as long as they are the appropriate thickness, and you don’t use too many.

Angular soft foot
Figure 2. Angular soft foot

2. Angular soft foot happens when a machine foot touches the base on either the side or outside of the foot. Because the other side of the foot is bent away, an angle is created between the base and the bottom of the foot. Angular soft foot is harder to diagnose because it occurs at an angle.

 

 

 

Squishy foot
Figure 3. Squishy soft foot

3. Squishy foot occurs when too many shims have been used to try to correct a soft foot problem or when an excessive amount of corrosion or debris has built up under the machine foot. This can happen when loosening the bolts during an initial misalignment check, which can release sediment and debris. It’s a frequent problem, especially if the machine’s bolts haven’t been tightened for a long time or if the machinery has been operating continuously and has rarely, if ever, had the bolts loosened.

 

External soft foot
Figure 4. External soft foot

4. External force is a soft foot condition caused by stress-induced external forces such as pipe strain, electrical connections, or severe misalignment combined with a stiff coupling. It can be hard to diagnose and can happen at any time during machine alignment.

 

This is the first installment of a two-part blog series about soft foot. In this blog, you learned about the symptoms, effects, and types of soft foot conditions. In the second post, you’ll learn how to diagnose and correct soft foot and what tools to use.  

Want to learn more about soft foot? Watch the Fluke Accelix Best Practice Webinar, “A hard look at soft foot: detecting machine frame distortion before it causes major issues,” which provides more in-depth information.   

4 ways to diagnose and correct shaft misalignment

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Technology and advanced tools are enabling maintenance teams to address shaft misalignment faster and easier

Misalignment occurs when two rotating shafts are not parallel to one another, whether it be an offset or an angular gap at the coupling. Machinery misalignment is extremely common in rotating machinery and is typically the root cause of machine failure.

The good news is that today’s technology makes it faster and easier to identify misalignment issues in machinery such as motors, pumps, blowers, and fans. Once it is diagnosed, we have better tools than ever to tackle these challenges, whether simple or complex, and achieve precise machinery alignment.

5 types of misalignment

Misalignment comes in different types and magnitudes. Experts generally agree on five types. Often the alignment issue is a combination of both an angular and offset misalignment, which is called a compound misalignment. Here are the five:

  1. Parallel misalignment happens when the two shafts to be aligned have centerlines parallel to each other, but the centerlines are offset.
  2. Vertical angle misalignment is a misalignment of two shafts in the vertical plane.
  3. Horizontal angle misalignment is a misalignment of two shafts in the horizontal plane.
  4. A combination of a vertical angular and offset misalignment occurs when one shaft is at an angle with another shaft, but both shafts still operate in the same vertical plane.
  5. A combination of a horizontal angular and offset misalignment occurs when one shaft is at an angle with another shaft, but both shafts still operate in the same horizontal plane.

You achieve alignment, under normal conditions, when the positioning of the rotational centers of two or more shafts is arranged in a single straight line, both horizontally and vertically.

shaft_alignment

What causes misalignment?   

These are the main triggers:

  • Settling of a baseplate resulting in soft foot
  • Shaft deformation caused by torsion during startup
  • Insufficient or poor-quality alignment measurements due to human error
  • Pipe strain that leads to parallel and angular misalignment
  • A thermal expansion that can make one piece of equipment move proportionally to another

Misalignment of shaft centerlines can give rise to shaft wobbling and significant vibration, leading to seal damage and bearing and coupling destruction. Identifying misalignment and the underlying causes is imperative if a machine is to reach its expected life.

4 ways to detect shaft alignment issues

  1. Diagnosis via laser shaft alignment system

Checking for shaft misalignment is often the first course of action when a machine starts having a problem. A laser shaft alignment system determines misalignment at the coupling and calculates the appropriate machine movement to be performed at its base. Laser shaft alignment systems provide real-time, accurate, and precise measurements that tell you exactly when alignment is achieved.

Laser systems can be either dual or single laser types. However, a dual laser alignment system is not as capable as a single laser system. Often, technicians find it hard to maintain the line to the detector when trying to correct an angular misalignment. Unfortunately, as the measurement distance increases, so does the difficulty. As a result, technicians must restart measurements that require them to stop, loosen the feet, move the machine, and retighten the feet—all with no guarantee that the detector will then be in range. With every repeat, process time increases, and the potential for error grows.

A single laser alignment system removes all of the dual laser system’s problems. It addresses most any shaft alignment challenge, whether it’s straightforward or complex, such as a vertical flanged machine with right-angle gearboxes. A single laser system leverages two optical detector planes in a single sensor, eliminating the technician’s need to stop, loosen, and retighten feet, or repeat measurements. Single-laser technology enables precise, swift completion of alignment tasks.

  1. Vibration analysis

Because of the speed at which modern data collectors can process triaxial sensor signals, vibration monitoring and analysis are becoming more commonplace. Every machine has its level of normal vibration, but there are instances when machine vibration increases or exceeds acceptable parameters.

Some of the reasons for vibration increases include looseness, resonance, unbalance, and of course, misalignment. Vibration is most often measured by using accelerometers, followed by proximity sensors or velocity transducers. Once these signals are acquired with a data collector and sensors, a vibration analyst can further diagnose the machine faults.

  1. Thermography

Thermography can provide early detection of alignment issues by identifying abnormal hot spots. Misalignment can cause an increase in friction within a machine and, therefore, an increase in temperature. A thermal imager detects and measures an object’s infrared energy and turns the infrared data into an electronic image showing the object’s apparent surface temperature.

  1. Oil analysis

When an abnormal condition or parameter is identified through oil analysis, immediate actions can be taken to identify the root cause or to act on the emerging failure.  Oil analysis can help detect misalignment by providing the general condition of an asset, oil, and contaminants. Oil analysis can be complicated; as a result, many organizations use specialty companies to do the work.

Misalignment causes a variety of symptoms and severe machinery issues. Implementing a preventive maintenance routine to verify precise machinery alignment will save money and time. By keeping shafts aligned, you also improve energy efficiency, increase uptime, and maximize production output.

Learn more about misalignment and how to correct it by reading this white paper.