Pruftechnik Blog

soft foot

The benefits of performing precision alignment at installation

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precision alignment

Measuring for precise alignment when installing a coupling driven asset is a smart and cost-effective way to help assure a longer machine life — right from the beginning of its active use.

Maintenance teams that measure alignment during initial machinery installation often see benefits in machine performance, efficiency, and health.

Using PRUFTECHNIK Adaptive Alignment systems can be especially valuable during machine installation because the results are highly accurate, getting measurements can be quick, and moving the machine may not be necessary.

Here are three alignment problems that can be minimized or resolved by measuring the alignment during installation phase.

1. Soft foot

Soft foot is a commonly used industry term describing distortion of the machine frame and “feet.” It occurs when a rotating machine set on its base, frame, or soleplate is off-kilter because one or more of its “feet” fails to make contact with all of the “foot-plates” on the frame.

It is the root cause of most alignment issues and a critical aspect to check before performing an alignment. Eliminating soft foot saves substantial time and money, especially since an estimated two-thirds of all rotating machinery have soft foot conditions. Addressing it right from the beginning typically makes subsequent alignments much easier.

Additionally, if the soft foot correction is successful, the bearing and motor typically have longer lives because the frame is no longer under tension and because there’s a better vibration signature when the soft foot is eliminated. Correcting soft foot improves seal and bearing life and reduces vibration—all of which contribute to a healthier machine.

Diagnosing and correcting soft foot when installing a machine enables maintenance teams to protect against further occurrences. The soft foot diagnostics tool in Adaptive Alignment systems provide a simple, convenient way to confirm the presence of soft foot and dirive and inform the user which type of soft foot and corrective action required to remove the soft foot.

2. Bolt-bound or base-bound

During initial machine installation, it’s essential to align the motor so there is some clearance in the bolt holes.

The most logical method for preventing a bolt-bound or base-bound condition is to install the stationary and movable machines properly before any piping, ductwork, or other attachments are added.

The absolute best time to use a laser shaft alignment tool to avoid this condition is during the initial installation.

3. Pipe strain

Pipe strain is likewise best addressed when machinery is first put in place. Pipe strain occurs when piping puts stress on the machine due to poor alignment of the pipes before they are bolted up.

The amount of this stress can be measured using the Live Trend feature of Adaptive Alignment laser tools. Measurements can also be performed by watching the pump during live-move or alternatively by taking an alignment measurement but instead of then moving the machine, unbolt the pipes, and taking another set of measurements and then compare the differences between the two sets measurement results.

The objective is to determine how much influence the pipes attached to the pump have on the alignment. It’s much more effective to address pipe strain when machines are being installed because teams can easily locate and correct pipes that are poorly aligned.

Using reports as a baseline for future inspections

Adaptive Alignment systems provide documented PDF reports about the condition of alignment and the presence of soft foot or pipe strain. Optional PC software is available for storing this information, making it easy to keep a running trend of machinery condition. This type of reporting tool is invaluable, especially in new installations, because it provides baseline data to reference when performing subsequent alignments.

Get more information about Adaptive Alignment at the PRUFTECHNIK.com website. Also, learn more about the Adaptive Alignment concept and feature set in PRUFTECHNIK systems by downloading the white paper on this webpage.

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.