Wind Baffle Drive Surface Hardness Measurements

Sloan Digital Sky Survey Telescope Technical Note 19980410

William Boroski and Stephen Bastian

Contents

Overview

Surface hardness measurements were made on the wind baffle azimuth drive surface and on the wind baffle azimuth drive rollers.  According to telescope design drawings, the drive surface is made from 1045 hot-rolled steel and the drive rollers are made from 1040 hot-rolled steel.  There are no surface hardening specifications or references on the drawings associated with the drive surface or drive rollers.

A portable EQUOTIP hardness tester was used for these measurements.  The measured Brinell hardness of the wind baffle drive surface was 178, which corresponds to 1040 in a normalized state.  The measured Brinell hardness of the drive roller surface that contacts the drive surface was 211, which indicates that the roller drive surfaces may have been slightly work-hardened. The Brinell hardness of the drive roller side faces and flange is 202 and 192, respectively.   Since the Brinell hardness of 1040 hot-rolled steel in its as-rolled state is 201, it appears that the roller drive surface is the only part of the roller that may have been hardened.
 

Hardness Measuring Device

Hardness measurements on the wind baffle azimuth drive were made on-site at Apache Point Observatory (APO) using an EQUOTIP hardness tester.   The EQUOTIP system, manufactured by Proceq SA, uses an impact device and the principle of energy measurement to determine the hardness of a metallic surface [1].   During a hardness measurement, the spring-loaded impact device is held tight against the test surface.  When the spring of the impact device is released, a spherical tungsten carbide test tip inside the device is impacted against the test surface.  The impact and rebound velocities of the carbide tip are measured and processed into a hardness value that is displayed on a digital indicator.   The displayed hardness value is converted into hardness values of other systems such as Brinell, Vickers, and Rockwell, through the use of supplied conversion tables. Prior to any conversions, corrections are made to the raw hardness readings to compensate for measurement variations due to the orientation of the impact device.

The accuracy of the measurement system is determined by how the measured values are used.   When using the hardness values obtained directly from the EQUOTIP readout, the average measurement deviation is 0.5% (+/- 4 units throughout the entire measurement range).  However, there is a loss of accuracy when converting the hardness value into static indentation hardness numbers, such as Brinell or Rockwell.  The mean conversion deviation ranges from +/-3% to +/15%, depending on the measuring range and the static method used.  One reason for the large variation is that there is no clear physical relationship between the various measurement methods.  A second reason is that the comparison of hardness values from different scales includes the measuring deviations of each of the methods.  For the measurements described in this report, the conversion deviation when converting from the raw hardness values into the Brinell hardness equivalent is +/-7%.

The calibration of the hardness tester was checked and verified before and after these measurements were made.  Calibration was checked against a standard supplied with the unit.   The surface hardness of the standard was 760 +/- 6.  The hardness measured by the tester was 760 for both calibration checks.
 

Drive Surface Measurements

Hardness measurements were made on two surfaces of the wind baffle azimuth drive assembly.  The hardness of the drive surface was measured, as was the hardness of the surface that will be in contact with the wind baffle guide rollers.  In both cases,  hardness measurements were made every 15 degrees around the drive circumference.  At each measurement location, three readings were taken and averaged to obtain the localized hardness at that point.  The average hardness of the entire surface was subsequently obtained by averaging all of the readings.

The average Brinell hardness for the drive surface and guide roller surface is given in the following table.
 

Table 1.  Brinell Hardness Numbers for the Wind Baffle Drive Surface
 
Location
Brinell Hardness Number
Drive Surface
178 +/-3
Guide Roller Surface
181 +/-6
 

From Machinery's Handbook, Vol. 24, 1040 hot-rolled steel in the normalized state has a Brinell hardness of 170, and in the as-rolled state has a Brinell hardness of 201 [2]. Within the accuracy of these measurements, it appears that the metal ring that provides the drive and guide roller surfaces has been normalized but not hardened.  The Machinery's Handbook defines normalizing as the process in which an iron-base alloy is heated to a temperature above the transformation range and subsequently cooled in still air at room temperature.  Carbon-steels are normalized to put the material into a uniform, unstressed condition such that the material will respond to further heat treatments.  Normalizing is also used to transform steel into a sufficiently soft state for machining.  For low-carbon steels, normalizing alone is sufficient.  For medium and higher carbon steels, the normalizing process is typically preceeded by an annealing process.  This two-part process is often called double-annealing.  As 1040 is a medium-carbon steel, it is possible that the drive surface material was double-annealed.  However, that fact cannot be ascertained from these measurements.  What is clear is that neither the drive surface nor the guide surface have been hardened.
 

Drive Roller Measurements

 Hardness measurements were made on each of the three azimuth drive rollers.  For each roller, measurements were made on the drive surface, the flange surface, and the front and back faces.  For each of these surfaces, the hardness was measured at three locations.  For each location, a set of three readings was taken and averaged.  There was good agreement in the readings for the three rollers, indicating that all rollers were manufactured in a similar manner.

The average Brinell hardness for the various roller surfaces is given in the following table.
 

Table 2.  Brinell Hardness Numbers for the Wind Baffle Azimuth Drive Rollers
 
Roller location
Brinell Hardness Number
Drive surface 211 +/ 4
Flange surface
192 +/- 3
Front/back face surfaces
202 +/- 6
 

The drive rollers are made from 1045 hot-rolled steel.  Brinell hardness numbers were not readily available for 1045, so values for 1040 were used for comparison purposes.  The Brinell hardness for 1040 in the as-rolled condition is 201, therefore it appears that the rollers as a whole have not been heat-treated.  It appears that the flange is the softest part of the roller, and that the drive surface has either been locally treated or has work-hardened due to contact with the drive surface.

 

Conclusion

Hardness measurement have been made on the wind baffle azimuth drive surface and azimuth drive rollers.  The results indicate that none of these surfaces have been hardened to any appreciable degree.   This is in agreement with the design drawings for these components, which do not list any specifications or requirements for surface hardening.
 

References

1.  Proceq SA.  Riesbachstrasse 57, CH-8034, Zurich, Switzerland.

2.  Machinery's Handbook, 24th Edition,  Industrial Press, Inc.,  New York  (1992).


Questions regarding this note should be addressed to Bill Boroski by phone at (630) 840-4344 or by e-mail at boroski@fnal.gov.
 

Last modified 02/23/99