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Ask The Search Engineer

Ask The Search Engineer

Measuring blades . . . Oven bearings . . . Encoder down under . . .

Dear Search Engineer: During the repair of aircraft turbine engines we have to grind the tip blade diameter of the compressor and turbine stages. I'm looking for a system to measure the tip blade diameter inside our new grinder without having to take the piece to a tri-dimensional machine. It has to be appropriate to the different diameters and lengths of the various engines. -J.G. in CA

Dear J.G.: It may be possible to integrate a vision system to image the blade tips in between grinding cycles. The camera could be extended into the grinding area during a pause in grinding, take the necessary images, and then be retracted to a safe location. I suggest contacting a vision system integrator to evaluate the application. Try this site (www.bentleyautomation.com) for an analysis of the application.

Dear Search Engineer: I'm looking for mounted or sleeve bearings for use on idler sprockets or rolls in ovens (350-500F). The standard graphite bearings are very speed limited. Do you know of any polymer or ceramic bearings that would work?
-P.V. in PA

Dear P.V.: EDT Corp. of Vancouver, WA makes sleeve (plane) bearings for both mounted and cylindrical applications for oven conveyors and idler sprockets. These bearings are greaseless, but are not suitable for unlimited speeds. There are many applications like this currently in service and some come with performance guarantees. The company can be reached at [email protected].

Dear Search Engineer: We have designed an actuator that uses a ball screw to provide a shuttle with linear motion between two stops. At the ends of travel are proximity sensors that signal the motor to turn off. The reciprocating shuttle must contact the stops at each end, therefore the sensor is only activated about 0.04 inch from the ends. This does not allow enough time for the motor/gearbox to stop spinning completely, resulting in a slight impact. We have incorporated a torque limiter to prevent this overload being transmitted into the system, however it requires frequent replacement due to wear. Are there any torque limiters that can be frequently "slipped" without wearing out? We do not need anti-backlash and the limiter could even disconnect completely, but it must re-engage automatically. -M.H. in NY

Dear M.H.: A more reliable and cost-effective solution could consist of using an electronic single shot device triggered by the stop contact. This device will reverse the direction of movement for a very short period of time, absorbing the impact energy.

G'Day Mate: I am looking for a rotary absolute encoder which gives the angular position to 18 bits accuracy (per turn) over an SSI or parallel interface. I have found many 17-bit encoders meeting this requirement, but not 18-bit. It seems that 17-bit is the best technology available using optical disk encoding. I have also found very expensive 26-bit encoders, but they use the proprietor's own protocol instead of a common one like SSI or parallel feedback. When I ask them why they cannot simply make the 26-bit version on an SSI or parallel bit interface, they said that would be impractical-can anyone tell me why? -F.D. in Australia

Greetings F.D.: You can use a Netzer absolute encoder with an angular position up to 19-bits per turn, or 20-bits with custom version. Check out Netzer (www.netzerprecision.com). Netzer encoder is an electric encoder, not optical encoder with three advantages: low cost, compact size, and more robust than optical encoders.

Dear Search Engineer: I'm looking for an inexpensive way to measure and convert a linear scale into a count-in other words, to measure the height of a stack of material with a known, common thickness and determine the number of pieces. We have considered using a weight count but there are too many variables, like sheet dimension, separation material, the variety of skids it is delivered on, material type, etc. Using a linear scale to make the physical determination and dividing the material thickness would yield a count. Doing this with an inexpensive, portable unit is the challenge.- J.B. in WA

Dear J.B.: It is easy to design such a device. You only need to match 0 on the scale with the bottom of the stack of material. The scale may have marked the number of sheets and the height, or just the number of sheets. Locate it behind the stack of sheets and read the proper number on the "scale" in level with the top of the stack. It can be possible to improve it with light positioned in front of the stack, so you will see the proper number on the scale above the shadow. These numbers corresponding to height are easy to generate in MS Excel. Copy the numbers to AutoCAD and print it to scale. You can also measure height with a tape measure and use the handy conversion chart from MS Excel. It all depends on the particular conditions and requirements. In addition, there are companies making linear measuring transducers that work by the extension of a string or cable. One such company is Celesco in Chatsworth, CA (www.celesco.com). One would have to make a small electronic readout device to work with these transducers. Another possibility would be to modify the electronics in an electronic tape measure of the type that can be purchased at Home Depot.

Dear Search Engineer: What national standard is used to define the metallurgical composition and the physical properties of 4140 steel?-J.B. in SC

Dear J.B.: There are several standards, depending on the shape and treatment of the product, including:

  • AISI 4140

  • UNS G41400, H41400, K11546

  • ASTM-A193, A194, A29, A322, A331, A506, A513, A519, A646, A752, A829, A914, A983

  • FED QQ-S-626C, Mil-S-5626C

  • SAE - 770, J404, J775

Dear Search Engineer: I am looking for a pressure sensor that can be inserted in a composite lay-up and will give feedback during a 380F and 100 psi cure cycle.-G.S. in CO

Dear G.S.: I assume the lay-up involves several layers and rather than measuring pressure you could measure force indirectly with either friction behavior or physical displacement of layers. Two different techniques come to mind.

  1. 1. You may be able to insert thin metal strips between layers and pull these out with a "fish scale." The force needed would be directly proportional to pressure. You would have to develop an initial experiment to correlate force versus pressure versus temperature.

  2. 2. Capacitec (www.Capacitec.com) makes commercial capacitance displacement sensors, which are essentially thin strips between .004 to .008 inch in thickness, which works at elevated temperatures. You would obtain a continuous readout of displacement between the sensor and one of the adjacent surfaces with micro-inch resolution for the .004-inch thick sensor. The .008-inch thick sensor is the equivalent of two .004-inch thick sensors giving you displacement to both adjacent surfaces. Again, you need to devise an initial calibration set-up and the sensors could be sacrificial.

Dear Search Engineer: My electromechanical application has less than 50 mA current at 12.8 and/or 5.0V for 1 million cycles. What kind of plating is required for AGNI contacts or phosphorous bronze or beryllium copper stamped moving contacts? I have heard multiple theories that fine silver plating is okay, and min 3-micron gold plating is required. What role is there for contact material and moving contact thickness? -K.D., Chicago

Dear K.D.: The problem you may run into is oxide build-up because low current will not burn it off. You may want to consider a Honeywell Microswitch (http://rbi.ims.ca/3854-523) for switching a logic signal of 5V through a 10K resistor. If you open the switch up, you can see that they use a wiping action to keep the contacts clean and a grooved pattern that allows oxides to be moved away from the contact surface. Silver contacts are generally considered the best in this application, as gold is generally a poor choice with wiping action, since it is soft enough to be worn away.

Dear Search Engineer: I'm an inventor working on a prototype of a complex product that requires up to 12 motors per unit. I'm not an engineer, so I need your help. If I were to seek quotes on fractional motors, what are all the specifications I would have to submit in order to receive accurate and meaningful responses? The motors will be the most expensive components of the product, so an accurate cost estimate is vital. The rpm and torque are the most critical features of the motors.-W.B. in CA

Dear W.B.: What is needed is the speed, torque, any dimensional restrictions, description of the application, operating environment, type of motor required (permanent magnet, universal, etc.), type of controller, duty cycle, and knowing if replaceable brushes are required. Also required will be the estimated annual usage and the type of termination required (leads, power studs, plug, etc.). There are several custom motor companies that can be found by a Google Search.

Dear Search Engineer: I use a multi-axle platform to carry heavy loads. To avoid problems during carrying, I need to know the load supported by the individual tires, as a system per axle will not give me the full solution. The system must be put into the tire, have a temperature correction feature, and deliver a signal to a central inflation system to maintain the inflation on each tire at the proper level. -C.B., Luxembourg

Dear C.B.: If keeping the platform horizontal is what's needed, then try tilt sensors in the X and Y directions for sensing purposes, which then control the inflation system.

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