Selecting the Appropriate Babbitt Alloy

Babbitt metals are either lead or tin based, and the metal makes excellent bearings that are easy to pour as a DIY project. However, before you pour your bearings, you must select the correct babbitt metal in order to ensure your engine or piece of machinery operates correctly and efficiently. If you know the composition of your existing bearing, you can purchase the exact same type of babbitt metal as a replacement. If you do not know the composition of your current bearing, you’ll need to choose a babbitt metal according to the bearing use, shaft speed and load factor.

Understanding Babbitt Metal Classifications

Babbitt metals are typically placed into two categories, including lead-based and tin-based. In general, lead-based babbitt alloys are great for low-speed and low-load applications, while tin-based babbitts are great for high-speed, high-load applications.

Lead-Based Babbit Metals

Lead-based babbitt metals are less expensive than their tin-based counterparts. However, they should only be used in low-load, low-speed applications. A good rule of thumb is to use a lead-based babbitt metal when the surface speed of the shaft is between 100 and 1,000 FPMs, and the load factor is between 100 and 500 psi. These specifications most often match what is needed to replace bearings in antique equipment, like the bearings inside antique car engines and compressor motors.

Tin-Based Babbitt Metals

Tin-Based babbitt metals are recommended for high-speed, high-load applications. Tin-based babbitt metals are more commonly used than lead-based babbitt metals because they are perfect for modern engines, compressors and motors. A good rule of thumb is to purchase a tin-based babbitt alloy when the shaft speed is between 1,000 and 2,400FPS, and then the load is between 100 and 2,000psi.

Determining The Surface Speed of Your Shaft

In order to choose the right babbitt metal, you will need to calculate the surface speed of the shaft. You can do this by multiplying the diameter of the shaft in inches by Pi (3.14) and the RPM and dividing the result by 12 (Pi*D*RPM/12=Shaft Surface Speed). This will give you the shaft’s feet per minute.

For example, if you have a 4-inch shaft traveling at 2,000 RPM, your formula will be 3.14*4*2000/12, which results in 2,093 feet per minute. This means you most likely need a tin-based babbitt alloy. However, you will still want to calculate the bearing load.

Determining the Bearing Load

Once you calculate the surface speed of your shaft, you’ll want to calculate the bearing load by dividing the weight carried by the bearing by the inside bearing’s inside diameter and the length of the bearing in inches. (W/ID*L=Bearing Load).

For example, if you have a bearing that is 6 inches long with a 3-inch inside diameter that carries a total weight-load of 200 pounds, your formula is 200/6*3, which results in a load of 11.11 pounds per square inch.

Choosing the Correct Babbitt Metal

Once you have your calculations, you will want to choose your new babbitt metal. This might be a difficult decision depending on the results of your calculations. In the examples above, the surface speed indicated that a tin-based babbitt would work best for the new bearing, but the load calculation indicated that a lead-based babbitt may be best. In this scenario, it is advisable to error on the side of caution and choose the tin-based babbitt alloy. This is because tin-based babbitt alloys work for high speeds, and they can have bearing loads of between 0 and 2,000 feet per second.

Belmont Babbitt Metals

Here at Belmont Metals, located in Brooklyn, we sell high-quality tin-based and lead-based babbitt metals that can be used for manufacturing replacement bearings for antique and modern engines. Our lead-based babbitts are great for low-speed, low-load applications and our tin-based babbitt alloys are great for high-speed, high-load bearing applications.

To learn more about our babbitt metals or to place an order, call us at 718-342-4900.