Improved Braking System for Automated Storage and Retrieval System Cranes

Photo of Eric Luna, Daniel Meyer, Nick Pedrazas Students: Eric Luna, Daniel Meyer, Nick Pedrazas

Sponsor: General Motors

Date: Fall 2007

Requirements:

Several requirements must be met in both the redesigned and replacement brake. The brake must be able to provide a static torque of 330 ft-lb in order to support the 2500 lb weight of the car body and crane platform. The brake should also be self adjusting to accommodate for wear of the friction plates over time and should be able to handle one million cycles before maintenance is needed. The time allowed for all of the improvements or replacements to be installed is three days. There are three cranes, each with motors and brakes for the vertical and horizontal movement, and each motor system is redundant, meaning that there is a total of twelve brakes to replace or improve in the time period allotted. The new brakes must also be compatible with the current system, meaning that they must be able to mount to the motor frame and shaft, and also run on the same computer controlled electrical input which controls the operation of the current brakes.

Problem:

To improve or replace Stearns' 82,000 series disc brakes on the Reliance motor for General Motor's Automated Storage and Retrieval System (AS/RS) cranes.

Solution:

Before improvements for the existing brakes could be completed, the causes of the failures for each brake were examined. From this data, recommendations were supplied to increase the longevity for each of the parts that were experiencing failures. To improve the solenoid, two thermal gap pads and an extra metal insert were inserted underneath the solenoid to improve the heat transfer rate from the solenoid. The other improvement to the solenoid is to add a brush system on the outside of the entrance where the plunger enters. This will prevent foreign particles from getting inside the casing and shorting out the solenoid causing it to arc weld the casing and plunger together. By improving the cooling of the solenoid, the rivets will no longer shear of due to swelling of the casing. The improvements for the gear that were found are to apply an NLGI grade 2 grease and to rotate the gear by a quarter turn during scheduled maintenance. These will increase the life of the gear by decreasing wear on the teeth. Next, the bolt holding the solenoid bracket to the base was increased from a 1/4” to 3/8” diameter more than doubling the axial and bending safety factors. To accommodate the new bolt a larger grommet was found that has lower stress concentrations. The total cost of all improvements total at about $205 for parts and two people for an 8 hour shift to update two brakes. The next avenue explored was finding a more reliable alternative braking system to replace the current Stearns brake. Based on knowledge obtained during the site visit and the Stearns failure modes, the focus of the search was centered on elimination of the solenoid and excessive moving parts. The search was narrowed to three possible candidates: electromagnetic disc brakes, electro shear brake, and drum brakes. After obtaining specific information for each brake, the results were tabulated in a Pugh Chart and judged on the following criteria: cost, reliability, ease of mounting, self-adjusting, and customer service. According to the Pugh Chart, the best option to realistically replace the Stearns brake is the Electro Shear brake from Midwest.

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