I’m just beginning to thaw out from my recent encounter with the extreme temperatures caused, so they say, by the polar vortex. This has reminded me of how extreme temperatures also can influence the measurements we try to make.
When most of us think about measurement environments, what generally comes to mind are pleasant laboratories with temperatures controlled to 68°F/20°C, plus or minus a degree or two. Or in the worst case, we picture a gaging shop with swings of temperature between 65° and 90°F.
When temperatures drop, maybe not to sub-zero but even to the 50s and 60s, there’s little most gages can’t handle. Even at 0°F there is apt to be little damage, though they may not work if they become frozen in place. It’s at the other end of the spectrum, when temperatures get extremely hot, that we can start to see gages becoming damaged. But there are steps that can be taken in design and materials that can keep gaging at work in some pretty hot places.
Sometimes a request like this will come across my desk: “Our research division needs dial indicators that will withstand working temperatures up to 250°F. They will be used in 10,000-hour tests, and must remain in the oven for the duration.” This customer was using an oven capable of cycling through a wide range of temperatures to test part performance, but this operating range is far beyond that of a normal dial indicator. A customized indicator with a glass dial and chrome body was supplied to meet the sustained accuracy requirements in the face of frequent thermal cycling.
Here are some of the considerations that go into making a dial indicator capable of such high-temperature applications:
• Crystal: One of the most obvious considerations is the crystal. Most crystals today are made from plastic blends or alloys that can withstand temperatures as high as 170°F. To accommodate 250°F, a glass crystal may be substituted.
• Bezel: Bezels are typically made of plastic or zinc. For extreme high-temperature gaging, a steel bezel is the most likely choice.
• Paints and coatings: Another consideration is the paint on the indicator and dial. Most won’t handle such extreme temperatures. We might consider a special temperature-resistant coating or, for the most extreme applications, none at all.
• Lubricant: Technically speaking, indicators are not lubricated. However, a small amount of watch oil is applied to the jewel bearings. At high temperatures, a special lubricant such as Molycote can maintain lubricity when other coatings would break down.
• Thermal expansion differentials: In dial indicators, there are some very tight clearances. Various materials used to make the indicator will expand and contract at different rates, so it’s important that the manufacturer use materials and clearances that will ensure performance of the gage over the entire temperature range of the test.
Clearances between brass bushings and the steel rack, and between the brass gears and the top and bottom steel plates that hold them in place must all be sized to eliminate potential binding or slop due to differential expansion or contraction.
The expansion characteristics of the high-temperature dial indicator should be provided to the user, making it possible to mathematically adjust measurements to compensate for different rates of expansion in the gage and the part.
• Spring performance: High temperatures may also diminish the force generated by take-up and pull-back springs within the dial indicators. As a result, better grades of steel and larger-diameter wire may be needed to ensure sustained performance of the gage. These types of customizations can accommodate measuring applications in temperature environments as high as 600°F.
No matter what direction the thermometer is moving, mechanical indicators can still provide an economical solution to some of the most difficult measurement problems.blog comments powered by Disqus