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Excel Expertise: Thermal Instability During Inspection

Q: Recently, a question came through the Excel pipeline concerning the effect temperature plays while inspecting components. Excel Expert, Ken Olson, paired with Excel’s Design Engineering team, offered this response:

A: Something to always be aware of when dealing with any critical crusher components that are machined to specific tolerances, is the fact that the temperature of the part will affect the actual measurements. Crusher components are typically cast or fabricated and made of steel, grey iron or bronze. All materials are subject to a physical phenomenon called thermal contraction or expansion, and each material type has a specific Coefficient of Thermal Expansion (CTE). These properties are taken into account during the actual machining process and all parts are measured to the NIST standard temperature of 68 degrees F. (20 degrees C.). Components that are at an ambient temperature that is greater or lesser than the NIST standard temperature will not measure up to the original (factory) measurements and thus may not be found to be within specification.

When measuring a part to size and tolerance for acceptance criteria, it is crucial to keep the Coefficient of Thermal Expansion (CTE) in mind. Though there are slight variations in CTE from one type of steel or one type of cast iron or one type of bronze to another, the following values can be used for the purposes of measuring major crusher components for critical dimension:

• CTE for Steel – 7.3 x 10 to the minus 6th.
• CTE for Cast Iron – 5.9 x 10 to the minus 6th
• CTE for Bronze equals 10.0 x 10 to the minus 6th.

The following is an example of a mathematical correction to dimensional calculations and based on an actual problem that occurred in the field with a crusher lower head bushing. The customer found that the o.d. of a lower head bushing was substantially (approximately .018” or .45 mm) undersize, and that all the previous lower head bushings also measured the same. I traveled to the mine site, located at 10,500 ft (3200 m) above sea level, to inspect the part in question. What I found was that the customer was bringing the bushing in from outside storage at an ambient temperature of several degrees under 32 F. (0 C.) into a warm shop and using micrometers at the shop temperature. The following is the correction calculation we used that was necessary to adjust for temperature:

Nominal Bushing Diameter: 42.650”

Measured Busing Diameter: 42.633”

CTE for Bronze: 10.0 x 10-6 (CTE for bronze)

Temperature differential 42 deg. F

CTE Correction Calculation: 42.650” x .00001 x 42 deg. F. = .018”

Please note that the diametrical expansion of a cylindrical body is the same for a given diameter as would be the expansion (or contraction) of a linear piece of the same length and material, subjected to the same temperature change. This has particular ramifications when it comes to a hot bronze bushing in a steel bore. Another term for CTE is Coefficient of Linear Thermal Expansion.

The best way to make sure you achieve accurate results while inspecting parts is to perform all inspections in a controlled environment using calibrated measuring equipment. If it is not possible to follow NIST standard and measure at a temperature of 68*F (20*C), then the mathematical calculation will be necessary to determine the actual dimensions of the part being measured. What’s more, it is important to keep in mind that it may not solely be the part that has been affected by thermal expansion. The materials that the specific instruments used for inspection may have also expanded or contracted. This is why it is always important to allow the measuring devices, micrometers, calipers, etc. to achieve the same ambient temperature as that of the part to be measured before taking measurements.

-Ken Olson, Excel Service Manager

Ken Olson200 Excel Expertise: Thermal Instability During Inspection

With over 30 years of experience in the mining industry and a degree in mining engineering, Ken Olson is a leader in the field. He helped develop the Raptor Cone Crusher line after joining Excel Crusher Technologies in 2004, and after joining Excel Foundry & Machine in 2012, he has been using his vast experience to greatly help the Excel team ever since.

Excel provides premium aftermarket components for more than a dozen different OEM brands. At Excel, we believe in delivering our distributors and customers quality products along with providing the very best in service and experience from industry experts.

 

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