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How to measure the clearance of Babbitt journal bearings?

by sherry•

If you’ve ever worked with rotating equipment—whether it’s a large industrial pump, a steam turbine, a paper mill dryer, or a heavy-duty compressor—you know how critical Babbitt journal bearings are to keeping that equipment running smoothly. As a Babbitt journal bearing supplier with 18 years of experience, I’ve seen firsthand that even a tiny miscalculation in bearing clearance can turn a seemingly minor maintenance issue into a catastrophic breakdown. Last year, we had a customer send us a failed bearing that caused their entire production line to shut down for 72 hours; the root cause? They had guessed at the clearance instead of measuring it properly during a bearing replacement. That’s why I’m writing this today: to walk you through exactly how to measure Babbitt journal bearing clearance the right way, with the hands-on, real-world tips I’ve picked up over decades of working with these components. No theory-heavy jargon, no shortcuts—just the process that saves our customers from downtime and costly repairs. Babbitt Journal Bearings

First, let’s get on the same page about what bearing clearance actually is, because this isn’t some arbitrary number. Babbitt journal bearings are split (most are, for easy installation) liners made of soft, tin or lead-based Babbitt metal that sits around a rotating shaft. Clearance is the tiny, intentional gap between the inner surface of the Babbitt liner and the outer surface of the shaft. This gap isn’t a flaw—it’s a feature: it holds a thin film of lubricant that keeps the shaft and Babbitt from touching, which prevents wear, reduces friction, and dissipates heat. Get the clearance wrong, though, and you’re in trouble. If clearance is too small, the lubricant film can’t form; the shaft and Babbitt grind together, melting the soft Babbitt metal, and you’ll get what’s called a “wipe” or a “scuff” failure. If clearance is too large, the shaft can wobble or shift excessively, which causes vibration, noise, and even misalignment issues that damage seals, couplings, and other nearby components.

The first step to measuring clearance is knowing what the correct target clearance should be for your specific bearing and application. That’s where most people skip a critical step: they grab a random bearing gauge and start measuring, without checking the manufacturer’s specifications (or, if you’re replacing a bearing, the original equipment manufacturer’s (OEM) guidelines). For our bearings, we provide a clearance table with every shipment, tailored to the shaft diameter and load rating of the bearing. As a general rule of thumb, the ideal radial clearance for Babbitt journal bearings is usually between 0.001 and 0.002 times the shaft diameter, but this can shift dramatically depending on the load, speed, and operating temperature. For example, a high-speed turbine (running at 10,000 RPM or more) needs a larger clearance to account for thermal expansion of both the shaft and the Babbitt liner. A slow-speed, heavy-duty crusher bearing, on the other hand, needs a tighter clearance to prevent excessive wobble under constant heavy load. Always confirm the target clearance before you start measuring—don’t guess.

Now, let’s talk about the tools you’ll need. You don’t need a fancy lab setup, but you do need the right tools to get accurate readings. Here’s my go-to kit for every bearing clearance measurement job:

  1. A outside micrometer, calibrated within the last 12 months, to measure the shaft diameter. This is non-negotiable—if your micrometer is off, your clearance calculation will be off. I keep two calibrated micrometers on my truck: one for shafts under 6 inches, one for larger diameters.
  2. Feeler gauges (also called feeler strips) of varying thicknesses, ranging from 0.001 inches to 0.015 inches, with narrow, pointed ends for getting into tight gaps.
  3. Plastigauge, a soft, plastic (polyethylene) strip that compresses evenly when pressure is applied—this is the most reliable method for measuring clearance on assembled bearings, and it’s something I recommend to every customer.
  4. A dial bore gauge, if you have access to one, for measuring the inner diameter of the Babbitt bearing. This is ideal for new bearings before installation, but Plastigauge works just as well for most field applications.
  5. Safety gear: work gloves, safety glasses, and a clean rag—you don’t want dust or oil messing up your measurements, or getting in your eyes while you’re working.

Before you start measuring, you have to prepare the bearing and shaft properly. If you’re working on a disassembled component, take your time to clean both the shaft surface and the inner Babbitt surface of the bearing with a lint-free cloth and isopropyl alcohol. Any tiny bit of dirt, metal shavings, or old lubricant will throw off your readings, or even scratch the Babbitt liner. If you’re working on an assembled bearing (installed in a housing on site), make sure the shaft is free of burrs or corrosion in the area where the bearing sits. Even a tiny burr the size of a grain of sand can make your measurement look larger than it actually is. Also, make sure the bearing housing is clean and free of debris—if the bearing is sitting crooked in the housing, your clearance measurement will be wrong.

Now, let’s get to the actual measurement methods. I’m going to cover the two most common, reliable methods: Plastigauge measurement (for assembled, installed bearings) and bore gauge micrometer measurement (for new or disassembled bearings). I use Plastigauge 90% of the time in the field, because it’s quick, and it gives a direct reading right on the job.

Let’s start with Plastigauge, since that’s what I teach our customers first. Here’s the step-by-step process I follow every time:

  1. First, confirm your target clearance and gather your tools. Make sure the shaft and bearing are clean and dry.
  2. Carefully remove any lubricant from the area where you’ll apply the Plastigauge. Oil will soften the plastic strip, leading to an inaccurate measurement. A quick wipe with a dry lint-free cloth is all you need.
  3. Take a piece of Plastigauge, and cut it to the length of the bearing’s inner surface (about ½ inch shorter than the bearing’s total length, so it doesn’t stick out the ends). Place the Plastigauge strip directly on the top of the shaft, along the center line—don’t twist it or fold it, just lay it flat.
  4. Now, carefully install the bearing cap (the top half of the split bearing) back onto the housing, and torque the cap bolts to the manufacturer’s specified torque. This is another critical step: over-torquing will compress the Plastigauge too much, giving you a false low clearance reading; under-torquing will leave the Plastigauge too loose, giving you a false high reading. I always use a torque wrench for this, and I double-check the torque setting against the OEM guidelines or our bearing specs.
  5. Once the bolts are torqued, carefully remove the bearing cap (don’t move the Plastigauge—if it shifts, you have to start over). You’ll see a flattened piece of plastic where the strip was.
  6. Now, take the Plastigauge scale that comes with the kit. Compare the width of the flattened Plastigauge to the printed lines on the scale. The number that matches the width of the compressed plastic is your actual radial clearance. Write that number down—this is the reading you’ll compare to your target clearance.
  7. If you need to, you can rotate the shaft a few times and take a second measurement at 90 degrees, just to confirm you didn’t have any uneven compression from the bearing cap. If the two measurements are more than 0.001 inches apart, you probably have a bent shaft or a misaligned bearing, and you should investigate further before proceeding.

The second method is using a dial bore gauge and outside micrometer, which is great for new bearings before installation, or for taking precise measurements on disassembled components. Here’s how that works:

  1. Measure the shaft diameter with your calibrated outside micrometer, at three different points along the shaft and at two different angles (0 degrees and 90 degrees) to account for any shaft ovality. Take the average of these six readings—this is your average shaft diameter.
  2. Set up your dial bore gauge: calibrate it by measuring a standard gauge block that matches the approximate inner diameter of your bearing. This ensures your gauge is accurate.
  3. Insert the dial bore gauge into the Babbitt bearing’s inner liner, and take the same set of measurements: three points along the bearing length, two angles. Take the average of these six readings—this is your average bearing inner diameter.
  4. To calculate radial clearance, subtract the average shaft diameter from the average bearing inner diameter. That gives you the total radial clearance (the gap between the shaft and the bearing). For example, if your average shaft diameter is 2.000 inches, and your average bearing inner diameter is 2.004 inches, your radial clearance is 0.004 inches.

Now, let’s talk about common mistakes I see all the time, because these are the things that lead to bad measurements and failure. First, using a dirty shaft or bearing. I once watched a mechanic skip cleaning a bearing for a power plant pump, and the Plastigauge had a tiny metal shaving under it that made the clearance read 0.006 inches, when the actual clearance was only 0.003 inches. He installed the bearing, and it seized up 2 hours later, costing the plant $120,000 in downtime. Cleanliness isn’t optional—it’s half the battle. Second, over-torquing or under-torquing the bearing cap when using Plastigauge. I always tell customers to use a torque wrench, and to follow the exact torque sequence (usually crisscross, to apply even pressure) listed in the manual. Third, using a worn or uncalibrated micrometer. If your micrometer has a zero error (like it reads 0.001 inches when it’s closed completely), every measurement you take will be off by that 0.001 inches. I calibrate my tools every quarter, and I check them with a standard gauge block before every job. Fourth, guessing at target clearance. Last year, a construction company called us because their new bearing was making noise. They had used the “0.001 per inch” rule, but their bearing was for a high-speed generator, which needs 0.0015 per inch of shaft diameter. Their clearance was too tight, and the Babbitt was already starting to scuff. We sent them a replacement bearing with the correct clearance, and they haven’t had an issue since.

Once you’ve measured the clearance, what do you do next? If the measurement is within your target range, you’re good to go—you can lubricate the bearing and install it. If it’s too small, you’ll need to either re-machine the bearing housing (if it’s an oversized housing) or get a new bearing. If it’s too large, you can either install a thicker Babbitt bearing or, for some applications, add a shim, but I always recommend replacing the bearing if clearance is more than 0.002 inches over the target, because excessive clearance leads to vibration and wear that will ruin nearby components.

As a Babbitt journal bearing supplier, my job isn’t just to send you a high-quality bearing—it’s to make sure it’s installed correctly, so it works as intended. That’s why I’ve spent years refining this process, because I’ve seen the cost of getting clearance wrong, first hand. Every bearing I ship comes with a clear clearance recommendation based on its application, and I’m always here to answer questions if you’re unsure about measurements on a job.

At the end of the day, measuring Babbitt journal bearing clearance isn’t rocket science, but it requires care, attention to detail, and using the right tools and methods. Skipping a step, guessing at numbers, or using dirty parts can lead to costly downtime, damaged equipment, and even safety issues. Whether you’re a maintenance technician, a plant manager, or someone who works with rotating equipment, taking the time to measure clearance correctly is one of the most important things you can do to keep your equipment running smoothly.

If you have questions about measuring clearance for your specific Babbitt journal bearing application, or if you need a custom bearing for your equipment, feel free to reach out to our team. We have years of experience working with all types of industrial equipment, and we’re happy to help you get the right bearing and the right measurement process for your needs.

Thin Walled Babbitt Bearings References:

  1. Stout, K. J., and Domblesky, J. P. (2000). Tribology for Engineers. CRC Press.
  2. American Society of Mechanical Engineers (ASME). (2017). Standard for Journal Bearings for Industrial Machinery. ASME Press.
  3. Neale, M. J. (Ed.). (2001). Tribology Handbook, 2nd Edition. Butterworth-Heinemann.
  4. Babbitt Bearing Manufacturers Association (BBMA). (2019). Installation and Maintenance Guidelines for Babbitt Journal Bearings. BBMA Technical Bulletin.

Wenzhou Zhengbang Bearing Co., Ltd.
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