Gleason Gear Jaw Metrics

Project Number: P15665

GLEASON GEAR JAW METRICS

ABSTRACT

The Gleason Works is a Rochester based company whose “principal activity is the design, manufacture and sale of machinery and equipment for the production of bevel and cylindrical gears” [1]. The manufacture of these gears often involves a gear blank (gear prior to machining) being lowered onto an arbor by a pair of steel jaws. If these jaws are machined improperly or damaged in some way they have the potential to tilt and damage the entire gear manufacturing machine costing Gleason and its customers thousands of dollars in repairs.

Gleason approached RIT to design and produce a gear jaw inspection station capable of identifying defective jaws for three of Gleason’s machine models before they are allowed into the field in order to prevent future damage and costs. The requirements for this station state that it must be able to accurately characterize any tilt or deflection in jaw surfaces with all measurements being within 0.001” of their true values.

Our group’s solution to this involved the creation of an adjustable jaw mounting structure with full control over the pitch and roll of a measurement plane. This structure allows an operator to ensure a parallel measurement plane to a granite surface plate. This system is used in conjunction with an adjustable height drop test indicator to obtain accurate measurements of all relevant jaw surfaces.

INTRODUCTION

This project was conceived due to several malfunctions Gleason machines have experienced in the field. When a defective jaw or pair of jaws is loaded into a Gleason machine it can have the effect of tilting the gear blank. The gear blank is typically lowered onto a circular metal arbor. When a tilted blank is lowered onto this arbor it can cause serious damage to the arbor or become lodged in the machine. When this occurs it can require a large rework operation to repair. Gleason roughly estimates that catching a single non-conforming jaw could save the company up to $10,000 in repair costs and lead to higher customer satisfaction. This is the motivation behind this project.

Gleason officially requested a measurement station capable of measuring all jaws used in three of their machines, the 400H, 130/210, and the 200/300 with the potential to accommodate future machine models as well. Gleason also requested a system accuracy of 0.001” meaning that all measurement error in the system must add up to no more than 0.001”. Gleason estimates that if a discrepancy of more than .030” is detected from one end of a horizontal gear blank contacting surface of a jaw to the other, the jaw will be unacceptable for use in the field.

Gleason has never designed a system for this purpose before, and a review of prior RIT projects showed no similar work. Based on this, all background research into this project was largely based on prior group member experience, and through the use of subject matter experts both at RIT and in the field.

The system was broken down into several subsystems, measurement and data collection, jaw securing and structure, feedback, and calibration. While some aspects of this such as the securing of the jaws had straight forward solutions, others such as measurement and data collection did not.

Several group members have previously worked with various metrology equipment in the field and this was the starting point for this project. The group brainstormed all of the various gauges and technologies we have worked with or heard of in the past and conducted extensive Internet searches to discover new technologies. It was immediately clear that some form of accurate calibrated gauge would be required to characterize the jaws. The initial research turned up several promising technologies;

1)Drop test and test indicators: Small calibrated linear or radial displacement gauges (both digital and mechanical) commercially available in a wide variety of lengths, accuracies, and prices. These gages are ideal due to their relatively low prices, versatility, and accuracy.

2)Height Gages: Mobile tabletop gages which measure the height of surfaces through the use of highly accurate calibrated linear encoders. Though ideal for their accuracy and ability to measure at varying heights, digital height gauges are more costly and their ability to measure height discrepancies over a single surface is questionable.

3)Linear and rotary encoders: Devices which turn linear and rotary motion into voltages which can be used to accurately record motion. Though incredibly accurate, such devices are typically expensive and would require the construction of complex systems to use which may already be commercially available (see Height Gage).

4)1D and 2D Lasers: Laser measurement devices which either give linear distances from gauge to surface, or provide a map of an entire two dimensional surface. These devices were found to be the most accurate option available but were found to be well outside of the project budget.

5)Coordinate Measurement Machines: Commercially available machine which consists of a flat granite table and accurate probes which can manually or automatically characterize the geometry of hardware. Though accurate and made specifically for applications similar to this project, these machines are expensive and would require modifications to accommodate this project.

With these ideas in mind, our group approached several members of the RIT staff as well as representatives from various metrology companies to refine these basic concepts into a functional measurement solution.

Research into the jaw securing subsystem was split into two further subsystems, a table to hold the weight of the device, and a jaw positioning system to position and secure the jaws to the various machines. The table research was straightforward and involved Internet searches to find a table which was large and sturdy enough to support all other hardware. The jaw positioning system research involved extensive communication between our group and Gleason to determine the jaw mounting geometry of the various machines involved in our project, as well as communication with professional toolmakers and subject matter experts to design a system which can be adjusted and calibrated to ensure accurate measurements.

Benchmarking and research into the feedback and calibration systems was unnecessary as the research mentioned above provided solutions for these without requiring additional work. Based on all of this research and learning our group was able to provide a functional design despite our lack of extensive experience in the metrology field.

PROCESS

The most critical step in the design process of this project was gaining a proper understanding of the customer requirements. The initial guidelines were vague, but after numerous meetings with the customer we were able to come up with a suitable set of requirements which enabled our design process. Some of the more critical requirements which impacted our final design are listed below.

1)Provides accurate measurements

2)Securely positions the assembly

3)Conforms to predefined budget

4)Accommodates three specific machines (GP400H, GP200/300, GP130/210)

5)Measures all critical surfaces

These customer requirements enabled the creation of objective measurable engineering requirements. Several of the design critical requirements are listed below.

1)Gage Error From Ideal Over Full Measurement Envelope ≤ 0.0004” (1.016e-5 meters)

2)Functional Prototype Cost ≤ $10,000

3)Jaw deflection when subjected to 20 Lb load ≤ 0.0004” (1.016e-5 meters)

4)Measurement System & Output Error ≤ 0.001” (2.54e-5 meters)

These requirements provided a concrete framework for our design efforts. By combining our aforementioned research and benchmarking with these requirements we were able to begin developing our ideas into physical concepts.

The jaw securing and structural system was the first to be designed. Several key assumptions were made based on subject matter expert input on machining processes, as well as the general engineering comprehension of our group;

1)No part can be machined perfectly. Every part created will deviate from the specified dimensions by some amount no matter how carefully it is machined. Similarly, error in one part can stack with errors in subsequent parts. In order to guarantee accurate and reliable measurements the system must be adjustable and capable of some form of calibration to remove these inherent uncertainties.

2)All materials deform. Even the strongest materials will deflect when subjected to a load or moment from a jaw (sometimes weighing up to 15 lbs or 67 Newtons). Due to the strict accuracy requirements of this system, efforts to maximize the rigidity and structural integrity of the system must be taken, and calculated deflections must be added to the final measurement uncertainties.

Knowing this, the group decided that the system had to be adjustable and capable of some form of calibration in order to remove the stacking of tolerances between parts. The critical parameter which we needed to establish in order to eliminate inaccuracies was a plane of measurement which was completely parallel to the plane in which our gages would travel. In order to do so, we took examples from various preexisting metrology equipment. After several iterations of ideas, the simplest method of adjusting the angle of the plane of measurement was established. We decided to use a series of bearings and thumb screws which can be turned to adjust the pitch and roll of the system. This enables complete control of the plane of measurement, and allows for periodic calibration of the system to ensure measurement accuracy. In addition to the adjustment screw and bearing system, a machine spring was placed opposite each screw to ensure system stability and reduce vibrations. This spring was sized to provide a moment to counter 80 lbs of force placed on any point in the system (this number was the result of anecdotal experience of a subject matter expert). The customer requested all parts be made from a specific steel alloy (AISI 8620) which has superior strength to low grade steels such as 1018. Based on the strength of the material and relatively low weight of the system only basic stress analyses were performed and shown to be largely unnecessary. Figure 1 below shows the final form of the adjustable jaw securing and structural system.

Figure 1: Adjustable Jaw Securing System

The next critical design step taken was to minimize the deflection of the system when subjected to the load of the jaws while also ensuring a proper jaw height, and minimizing the cost of the materials required to make the system. We designed a vertical structural member known as the “central column” to support the jaws and ensure they are at the proper height. To calculate the system deflection, a conservative bending analysis was performed on this central column. The customer gave a maximum jaw weight of 15 lbs and stated they could be up to 18 inches in length and height (67 newtons and .457 meters respectively). Using these parameters, we assumed the full 15 lb load was applied at the full 18 inches from the central column. The resulting moment was applied over the full 15 inch (.381 meters) length of the column. This analysis returned a bending angle which was then converted into a deflection over an 18 inch jaw. This process was iterated with thicker column widths until an acceptable 18 inch deflection was reached. With a 2 inch thick column a maximum deflection of approximately .00027 (6.780e-6 m) inches could be reached (with only one jaw mounted). While this number is nearly one third of our allotted .001 inches, it is the most significant source of error in the system that cannot be calibrated out and thus is acceptable. The full bending analysis may be found on our group website [2].

In order to mount the jaws to the central column, a versatile series of machine interfaces had to be designed. In order for our system to accommodate more than one Gleason machine, interchangeable machine interfaces were required. Our solution to this problem was to design a series of three 6.5” x 11” plates. These plates can be bolted directly to the top of the central column and mimic the geometry of the three specific Gleason machines we were asked to accommodate. These plates are exact replicas of the Gleason geometry, but the tolerances of our plates are tighter, and require surface grinding of all jaw locating features to ensure minimal machining error. The dimensions of these parts are among the most critical dimensions in the entire system. If these dimensions were out of spec, the system would be difficult if not impossible to calibrate due to improper interfaces between the plates and jaw or calibration hardware. Additionally, angular mismatches between left and right jaw mounting surfaces would negatively impact system accuracy. Special care was taken with these parts during the machining process as described below. Figure 2 shows the three completed interface plates.

Figure 2: Machine Interface Plates

Finally a table was selected based on its ability to hold the weight of our system, and a base plate was designed to place the central column and adjustment system on to prevent unnecessary damage to the wooden table.

The second system to be designed was the measurement and data collection system. The primary assumptions needed to enable the design of this system were as follows.

1)No gauge is completely accurate. All gages and metrology equipment will have some error. Though this error is unavoidable, it can be minimized through the use of properly calibrated equipment.

2)Error in metrology equipment stacks with other metrology as well as other mechanical tolerances and must be tracked to ensure the total system accuracy is within the acceptable limits.

These assumptions guided our group towards picking the most accurate measurement equipment possible, starting with our granite surface plate. Granite is a standard device used in the metrology field based on its ability to hold extremely tight flatness and surface profile tolerances. Our group selected a granite plate based on the size of our measurement envelope (approximately an 18 inch square), a customer request for additional storage space, and its overall tolerances. We selected an 18” x 24” grade AA granite surface plate meaning that the overall surface flatness tolerance is within 0.000075” over the full surface of the plate. This is less than 10% of our total system accuracy allowance.

After choosing the granite plate, we selected a gage. In order to measure several points on a surface and determine its profile and angularity, we knew we needed a device with the ability to accurately measure vertical distances. Several of the devices mentioned above were considered, however it was found that laser devices and CMM machines were outside of our budget, linear encoders are expensive as well and would require pairing with a second indicator to get useful measurements, and finally, normal test indicators are rarely accurate enough, and provide additional complexity due to their angular deflections. Based on these facts, a simple digital drop test indicator was selected as our gauge of choice. After consulting with a representative from the Starrettcompany, we decided that a Starrett 2900-2 drop test indicator was the best fit for our project. With an accuracy of 0.00012” (0.003 mm) this indicator takes up around 10% of our allotted accuracy requirement. As this gauge does not come with any fixturing, our group also had to design a vertical support structure to hold the measurement device steady during the measurement process. By coincidence we were able to salvage a modified Starrett 755-24 height gauge from RIT storage, and modify it to hold the indicator. This eliminated the need to design further support. The full measurement system can be seen in figure 4 below.

The feedback system design went hand in hand with the selection of a measurement device. Most devices we looked at had electronic outputs which were capable of sending the gage readings directly to an excel document. We purchased an output cable for this very purpose with an additional foot pedal to prevent an operator from shifting the system by pressing a button over the table. The output cable plugs into a computer which Gleason will provide via a USB port.

The calibration system was the final system to be designed. The purpose of this system is to remove all unnecessary system error. This system works in conjunction with our screw and bearing adjustment system described above to ensure the plane of measurement is perfectly parallel to the plane created by the interface plates. In order to accomplish this it was clear that flat plates similar to gage blocks would need to be designed to mimic the locating features of Gleason jaws, but with tighter tolerances. The basic idea was to create flat surfaces perpendicular to the interface plates and ensure their squareness to jaw mounting surfaces with careful surface grinding and machining techniques. These “ideal jaws” if machined properly would allow an operator to confidently calibrate the system to ensure that if a completely conforming set of jaws were placed in the system, zero deviation in height would be detected over the entire system measurement envelope. In order to use the calibration plates, an operator must align the locating features of the calibration plates with the interface plates, bolt the two together, and tram the drop test indicator across the top of the calibration plate until zero deflection is measured across the entire surface. It should be noted that the surface grinder used to create the finished surfaces of the parts has a flatness tolerance of approximately 0.0002” and that this number must be added to the overall system accuracy. An example of a calibration plate can be seen below in figure 3.