Spacer Shaft Alignment: Common Pitfalls and How to Avoid Them

A spacer shaft connects two machines using two couplings and an intermediate shaft. They are commonly used in pumps and compressor setups in many industries. Maintaining alignment is critical for machine health; however, many teams struggle with proper spacer shaft alignment because of their extended length and dual-coupling configuration.

This article will explain spacer shaft alignment, outline common challenges teams face when aligning spacer shafts, and provide tips for easier, faster spacer shaft alignment.

What Are Spacer Shafts?

Spacer shafts are intermediate shafts that connect driving and driven components. A spacer shaft is defined as a shaft that is longer than its diameter. It typically has a flexible coupling on each side to accept offset.

Spacer shafts have many applications. They increase the gap between machines to make them easier to work on, leaving more space for machine disassembly when necessary. More often, teams place them between machines that experience large amounts of thermal growth or between machines that run at top speeds. Since machines coupled with spacer shafts can accept more misalignment than those directly coupled together, spacer shafts allow higher shaft alignment tolerances.

Spacer Shaft Alignment Tolerances

Alignment tolerance charts show tolerances based on the rotations per minute (RPM) of a machine. The higher the speed of the machine, the closer the alignment must be to avoid damaging the equipment.

The chart below shows typical spacer shaft alignment tolerances based on RPM. While the spacer shaft alignment tolerances appear to be tighter than typical machine-to-machine alignment tolerances, the numbers shown are the amount of misalignment that is acceptable per 100 mm or one inch of the spacer shaft.

Why Spacer Shaft Alignment Matters

Over half of failures in rotating equipment can be traced back to one single cause: misalignment. Even though spacer shafts allow a higher tolerance for misalignment than direct coupling, it’s still important to maintain alignment within the recommended tolerances.

Misalignment can lead to premature wear on bearings and seals along with coupling stress, which increases maintenance costs. While a spacer coupling can help protect the machines by absorbing some of the stress caused by misalignment, too much misalignment can damage the spacer shaft, coupling, or other machine components, ultimately leading to downtime. It can also reduce efficiency and increase vibrations, increasing utility costs.

Spacer Shaft Alignment Challenges

Spacer shafts range in length from a few inches to several meters long. The length, in addition to the multiple couplings, can present challenges during the alignment process, especially when aligning very long spacer shafts.

Spacer shaft alignment with traditional dial indicator methods is particularly challenging due to the distance between machines. Since the weight of the bracket causes sagging over the distance of the spacer, the sag must be calculated and accounted for in the readings from the dial indicator. It’s common to miss or miscalculate this step, so it can be very difficult to achieve spacer shaft alignment with dial indicators.

Using a single-laser, dual-sensor alignment system, like the Prüftechnik RotAlign, OptAlign, or ShaftAlign eliminates the alignment challenges caused by sagging dial indicator brackets. The laser can cross over 10 meters without sagging, delivering precision even over long spacer shafts.

Best Practices for Spacer Shaft Alignment

Proper spacer shaft alignment is foundational for ensuring the longevity and efficiency of rotating machinery. There are a few spacer shaft alignment best practices that every technician should know to overcome common alignment challenges.

Before performing alignment, input target specifications for thermal growth and offset tolerances for higher accuracy during the alignment. Some laser alignment systems have a built-in feature that gives real-time feedback on machine adjustments. One example is LiveMove from Prüftechnik, which lets technicians know once they’ve achieved alignment within tolerance, even over long distances, and shows how the machine movement impacts alignment.

Major misalignment over long distances of a spacer shaft can mean that even laser systems have difficulty keeping the laser and the receiver in range of each other. One benefit of laser systems over dial indicators is that some have a feature that allows teams to recenter the laser if it goes out of range of the sensor. This allows measurement of the initial misalignment without requiring pre-alignment or multiple measurements.

If the distance between the couplings on either side of the spacer shaft is too far, or if the distance is obstructed by other machines, use the machine train alignment feature to treat the spacer shaft as a machine with no feet. This allows alignment by using a set of heads on each coupling and simultaneously aligning the entire machine train, which in this case consists of two machines and the spacer shaft between them.

For long-term reliability, you should trend alignment results over time. Laser systems with cloud connectivity can simplify this process by storing historical alignment data, showing how alignments change. This allows technicians to find and address the root cause of recurring misalignments and improve alignment processes and timing.

Step-by-Step Guide to Spacer Shaft Alignment with a Laser Alignment System

Aligning a spacer shaft is a fairly straightforward process with modern laser alignment tools. Here’s how to do it in six steps:

1. Prepare equipment: Ensure the machine is isolated and safe to work on. Correct soft foot issues using shims to eliminate base rocking. Input target specifications for thermal growth and offset tolerances into the laser system to account for operational expansion.
2. Mount laser system: Install a single-laser, dual-sensor system with one sensor on each coupling hub. Ensure the laser beam is within range (features like InfiniRange on Prüftechnik tools can recenter if initial misalignment is severe).
3. Take initial readings: Activate the laser system to measure the current alignment condition across the spacer shaft. If needed, use the machine train alignment feature to treat the spacer shaft as a component with no feet, capturing data from both couplings simultaneously.
4. Adjust alignment: Move the machine (vertically with shims or horizontally with jacks) while monitoring the laser display to bring alignment within tolerance. Features like LiveMove on Prüftechnik tools give real-time feedback for precise results.
5. Verify and finalize: Recheck measurements after adjustments to confirm the shaft is within tolerance. Ensure the laser beam remains stable over the distance, adjusting with InfiniRange if needed.
6. Document and trend: Save alignment data using cloud connectivity to trend results over time, identifying patterns or recurring issues for future optimization.

Final Thoughts on Spacer Shaft Alignment

Spacer shaft alignment is critical for maintaining the reliability and efficiency of rotating machinery, especially in industries that rely on pumps and compressors. Misalignment leads to costly wear, vibration, and downtime, but moving beyond traditional dial indicators to advanced laser systems marks a significant step toward efficiency and addresses challenges like bracket sag and long-span inaccuracies that occur with traditional alignment methods.

When performing spacer shaft alignments, it’s important to have a solid understanding of tolerances and how the length of the spacer shaft impacts tolerances. Laser alignment systems from Prüftechnik include built-in tolerance calculators and can calculate spacer shaft alignment tolerances based on the information you input into the tool. This helps avoid confusion and miscalculations and ensures you achieve alignment within tolerance based on your unique machine configuration, no matter how long or complex it may be.

Single-laser systems from Prüftechnik are the easiest alignment tools to use for spacer shafts, since they can accommodate even large misalignments over long spacer shafts using the InfiniRange feature. With onboard workflows and features like LiveMove and multi-coupling alignment, Prüftechnik laser alignment systems are built to simplify every alignment and achieve precision alignment even in the most challenging situations.