Meeting the Challenges of Measuring Large Parts

August 10, 2013

Manufacturers of large gear inspection equipment have to meet a number of new challenges with respect to physical size, moving masses, and thermal effects.


Overall Equipment Layout
Even though the measuring principles on very large equipment are the same or similar to small ones, it would be wrong to expect identical assemblies with simply larger dimensions.
Before going into detail, the following illustration shows the main assemblies of a large state-of-the-art gear inspection machine:

Figure 1

Legend:
1. Z-Column                                          9. Support for grinding burn detector periphals
2. Probe head                                      10. V axis (1.radial axis for positioning)
3. Special probe extension arm              11. Support for surface roughness measuring unit
4. Y axis (2.radial axis for measuring)     12. Active air leveling/shock absorber elements
5. Z slide (vertical axis)                         13. Equipment base plate
6. X slide (tangential axis)                     14. Rotary table (C axis) housing
7. Exchange disk (probe) magazine        15. Workpiece support, base element
8. Magazine clamping                            16. Workpiece support, extension arms (spider)


Mechanical Set-Up of a Modern Large Gear Inspection Machine
A modern large gear inspection machine incorporates granite guideways with air bearing slides, which is a benchmark for the industry. Hydrostatic rotary tables with direct drive motors provide the precision required for measurement of large diameters.

Inspection equipment should be installed close to the production facilities. Due to equipment size and location, workpiece transportation methods and paths often lead to a less-than-desirable working environment. To meet this challenge, large gear inspection equipment must have long-term stability, temperature compensation, and way covers.

Granite as the Overall Material, Air Bearings
For many decades, granite has been used for highly accurate CMMs and other measuring instruments. It is the perfect material for accuracy-determining components of industrial measuring technology for several reasons:

• Due to the age and origin, granite is free of internal tensions and thus externally stable in the long term
• The thermal expansion coefficient, i.e. the material-caused length variation in temperature fluctuations, is only about half that of steel and a quarter of aluminum
• It is highly wear-proof, pressure-proof, and deforms 25% less under load than an identical aluminum component, at a higher specific weight of only about 5%
• Granite is corrosion-resistant, non-magnetic, and has ideal damping characteristics
• Correct material-adjusted processing makes it possible to produce nearly pore-free, flat, and level surfaces that provide the perfect basis for air bearing guides—a technology that was a prerequisite for the ultimate accuracy of coordinate measuring equipment.

For those reasons, the base plate (13) and all axes (1, 4, 5, 6, 10) consist of this granite material with identical thermal behavior—a major advantage compared to cast iron, steel, aluminum, or a combinations thereof. The base plate should be located in a pit that enables level access to the walkable part of the plate and either rest on a suitable foundation and/or on active intelligent air leveling and shock absorbing elements (12).

On the granite guide ways, air bearings are used in all linear measuring axes (X, Y, Z). The positioning V-axis runs on ultra-precise resin-coated mechanical bearings for long term accuracy.

Axis Arrangement, Probe System
There is a special demand for ultimate high accuracy when measuring small, medium, and large parts on the same machine. This affects mechanical design, controller, and measuring head, and necessitates linear and volumetric compensation of axes characteristics by means of laser and masters. The axes arrangement is shown in Figure 2.

Axis V (10) is the primary radial axis and serves for prepositioning the X-Y-Z slide system in a range, where the subsequent measuring task can be carried out entirely by means of the linear axes X, Y, Z and rotary axis C (14). Radial prepositioning avoids a longer Y (4) axis that might tend to droop because of its own weight and would necessitate stronger Z slide mechanics (5) and drive. The V and Y axes position the probe radially.

The long linear tangential X (1.5m) axis allows positioning the probe in extreme positions. This is useful when measuring non-gear components and offers an additional location for a second probe changer column.

The very long vertical Z axis (2.1m) offers a huge measuring envelope, considering the possible travel of X and V+Y. External or internal gears with large face width and double helical gears with extreme sizes (as well as long shafts) can be easily accessed.

In order to achieve the demands for repeatability and accuracy in the whole measuring envelope, the machine has a complete corrective matrix determined by means of laser systems, certified masters, and step gages.

The probe system also has an important influence on the performance and the accuracy of a large gear inspection machine. The SP80 Renishaw scanning probe system has proven to be the ideal probe head, offering high resolution and high accuracies even when using long/heavy probe configurations.

Probe extensions up to 700mm in Y or +/- Z extend the measuring envelope significantly.
For large gears, the use of automatic probe changing is strongly recommended.

Weights and Masses, Rotary Table and Rotary Encoder
Measuring large gears of 5.5 to 6 meters in diameter, weighing up to 40,909 kg (90,000 lbs), demands special mechanical solutions for the rotary table and specific control measures with respect to polar inertia.

In order to provide long-term equipment performance and stability, the required very large rotary table weight capacity demands a table bearing with virtually limitless life, due to no metal-to-metal contact between the bearing elements. A well proven, technically advanced solution is a hydrostatic rotary table normally used in large gear grinding machines.

Beyond the part polar inertia data stated in its drawing, the inertia of the equipment's workpiece support has to be considered. In order to determine the actual conditions, the equipment needs to be capable of automatically tuning the acceleration/deceleration ramps in order to gain accurate and repeatable measuring results.

The rotary encoder being used for this large application is of special design with a large basic diameter and two reading heads, utilizing Double Scale Interpolation for enhanced accuracy.

Thermal Effects and Compensation
The larger the component and the longer the inspection time, the larger the change in size as a result of temperature change. By using granite for the machine guideways, this change is minimized due the low coefficient of expansion.

To compensate for deviations caused by temperature change, the equipment needs to provide several temperature sensors for the machine and workpiece, and corresponding algorithms to consider the temperature effects on the evaluation of measuring data.

Control of Equipment Functions
The systems should offer to the operator a utilities screen to control all necessary functions for running the machine. For safety reasons, the controller software is password-protected to avoid wrong inputs that result in uncontrolled movements of the axis (Figure 3).

The RP Utilities offer all necessary functions and give full support to the operator while running the machine. Some of the push button functions are as follows:

• Drive to reference points
• Standby position
• Probe information
• Move probe to position
• Automatic alignment
• Start measurement

Special Safety Features
Large gear inspection machines must provide additional safety features compared to small or medium size machines to avoid accidents during the measurement process (Figure 4).

The solution is the use of an intelligent laser system to control the area around the machine. The system is PC-based and the software allows the user to program different safety zones and exclude fixed areas like room columns. The system is connected with a signal light to indicate the present status of the machine. Layers of pre-warning signals proceed the last warning with a horn and, finally, an emergency stop will occur.

Ergonomics
When considering the unique demands of large gear inspection, operator ergonomics must be considered in the design. This includes:

• Accessibility of areas where operator intervention might be necessary, walk-able zones on surrounding floor height
• Clear working space and measuring envelope for ease of mounting devices and workpieces
• Control of equipment functions outside the standard routines
• Safety features

Accessibility
Operators should have easy and safe access to all areas of the machine whenever and wherever an intervention is needed for part loading, probe set-up, etc. (Figure 5). This can be fulfilled be means of walkable zones of all areas (including rotary table) on the machine, as well as lowering the machine base to a convenient level).

A movable operator station with status light includes large monitor, keyboard, mouse, joystick panel, and panel for laser safety system in a very ergonomic way.

Clear Working Space
Loading large gears up to 5.5m in diameter and up to 40,909 kg (90,000 lbs) requires the full attention of the operators. Therefore, the machine should offer maximum space to avoid collision and move to a parking position with the inclusion of a “Go to Parking” button (Figure 6).

To avoid any damage to the machine when lowering the part to the rotary table, the machine base sits on mechanical blocks during the loading procedure. After finishing the air leveling elements underneath of the base plate go back “on air” and the whole machine and is lifted.

PC-Hardware (Controller) and Software
In order to cover all measuring tasks (cylindrical and bevel gears, splines, tools, shafts, camshafts, crankshafts, worms and worm wheels, rotors etc. as well as housings and other 3D-parts) a state-of-the-art PC-based controller and a complete software package for data input, evaluation and output of results is required (Figure 7). The Geco-controller is one of the few control devices in the world developed specifically for this type of measuring equipment, with complete real-time integration of the sensor technology into the control.

Special Features
Leveling Table (Spider) with Cross-Slide and Software Supported Alignment
Large workpieces (externals or internals) require intelligent operator-friendly solutions for mounting and centering the parts to a position to run the final fully automatic procedure to determine the axis of the workpiece (Figure 8).

The procedure to mount and center a part (up to 90,000 lbs) is as follows:

• Mount three “fix points” on the table in positions provided by the machine using built in utilities
• Load part on the leveling table against or near fix points for pre-positioning
• Automatically probe actual position of part to get information on the screen on how to move the cross-slide
• Move cross-slide axis by means of monitoring the actual position on the screen, shown by the green rough- and fine-tuning bars, to position the part to the centered position

Grinding Burn Test (Barkhausen)
Typical analytical gear inspection can verify geometrical specifications of a part, but cannot look into the teeth nor deliver any data concerning the crystal structure of the part's material.  Grinding burn detection can be provided by use of an automatic Barkhausen-type system. The equipment provides a console (9) for accepting an evaluation unit of a grinding burn detector.

The special probe is mounted to a regular holding plate of the scanning probe and can be operated in a fully automatic mode, including probe changing.

Control of the process is via PC of the gear inspection equipment. All software for grinding burn test and evaluation is running on this PC (Figure 9).

Surface Roughness
In case surface roughness is inspected, an optional support (11) for the measuring equipment can be mounted to the X slide (6). The evaluation unit is close and easy accessible to the operator (Figure 10).

Customized versions: One size does not fit all. Gear measuring machines for workpieces with diameters of 1600 mm and larger should fulfill all individual demands of the customers and special requests of the operators. To provide this customization, R&P Metrology GmbH has developed modular design by which individual machines are configured:

• Individual design and size of base plate to fit into existing foundation or facility
• Several options for vertical measuring lengths
• Rotary tables with different load capacities using air bearings or hydrostatic bearings
• Variable counter support (tailstock) lengths and fixed or movable counter positions
• Intelligent solutions for centering, holding and clamping workpieces
• Complete software library including 3D-software packages 

About The Authors

Dwight Smith

is vice president at Mitsubishi Heavy Industries America Inc. Machine Tool division. He has more than 25 years of experience in the gear manufacturing industry, including metrology, analysis, and project management. Smith serves as a committee chairman for AGMA, and he is an instructor for the AGMA Basic Gear School.

Hans-Helmut Rauth

is president of R&P Metrology GmbH. His long career in gear metrology began in 1981 at Höfler Gear Inspection and continued with Zeiss IMT, Klingelnberg- Höfler and Wenzel GearTec.