Intech Power-Core Thermoplastics Engineering Blog

Anti backlash gear design at the heart of clear tomography imaging

Posted by Alexander Bartosch on Sep 21, 2012 1:58:00 PM

 

Special gear design is at the heart of clear image in multi-axis tomography device.

For the rotational machines used to gather dental X-ray images, motion chatter can produce a fuzzy image, which is not suitable for diagnostic purposes.

Panoramic radiography is a branch of rotational tomography where the creation of images are through the movement of a source and receptor in such a way as to cause the foreground and background structures to blur, leaving a defined focal trough.

As a panoramic radiographic device, the Vantage Panoramic X-ray System, designed and manufactured by Progeny, Lincolnshire, IL, incorporates a DC X-ray source, CCD digital receptor, distributed processing circuitry, and an LCD touchscreen control panel for ease of use. What makes the Vantage system unique is that it is adjustable to the patient’s height via a motorized, 3-speed, telescoping column. The use of multiple lasers is to locate the patient and configure the device to the patient’s morphology. In addition, a workstation coordinates the individual processors.

The system incorporates an overhead, swing arm (lateral Y-axis) that supports a C-arm, which is the rotating member that moves around the patient’s head. The C-arm includes a tube-head, which produces the X-ray beam, and a removable CCD sensor, which is the digital image receptor. If this arm does not operate smoothly, a distorted image results.

The swing arm pivots on bearings located in the mounting casting fastened at the top of the column. Producing its motion is a ball screw drive, one end of which connects to the mounting casting and the other end to the swing arm. A step motor mounts at the column end. Both mechanical connections of the drive assembly are through ball bearing assemblies.

Suspension of the C-arm is on a pair of bearings mounted to the underside of the X-axis translation plate. The C-arm casting incorporates a 10" ID internal tooth ring gear that meshes with a pinion gear on a step motor mounted on the stationary X-axis translation plate. The motor is spring-mounted to maintain positive mesh and to minimize slop. The internal ring gear and pinion are sized and shaped to engage on the inside surface of the C-arm. With activation of the motor, the stationary pinion engages the teeth in the internal ring gear causing rotation of the C-arm.

The engineering team at Progeny worked with Intech to help design the C-arm casting and its interface with the gear drive for the C-arm’s rotation. Design of the company’s Power-Core products is specifically to reduce noise and vibration and run without lubrication, an important factor (a must) for medical equipment designers. Intech components are far lighter in weight than metal parts and offer longer life (less wear) and lower maintenance costs. Intech engineers used a proprietary gear load/life calculation to verify that the gears designed into the dimensionally restricted place would last at least 8,000 hours of operation or about 15 years in field use.

The challenge was to design a backlash free gear to produce a steady rotational movement of the image producing components. There was no room to employ the traditional split gear design. To eliminate backlash, installation of a spring, on slight angle relative to the axis connecting the gear centers attached to the pinion, pulls the pinion toward the 10" ID internal ring gear. The spring arrangement did eliminate the backlash, but caused the gear teeth to bottom out, resulting in chatter. The chatter registered on the X-ray image.

The precision gearing for the Vantage Panoramic X-ray System uses Intech’s Power-Core nylon materials for reduced chatter, resulting in clearer images from the system.

Drawing on its expertise in gear design, Intech engineers designed and precision-machined the pinion and the internal gear to incorporate a special contact surface, which allowed the components to control the center-to-center distance between the inner tooth gear and the motor pinion. Adding the center-to-center distance management element presented a method for precise gear positioning in the mesh, and drastically reduced system vibration generated by the spring force and the resulting bottoming out of the gear teeth in the earlier design.

This configuration provides precise control over gear mesh vibration and backlash, resulting in high image quality in both a clockwise and counterclockwise rotation of the C-arm. It also adds a robust design element, which helps to increase product life so that image quality does not degrade with component wear and tear. With no fuzzy imaging due to chatter, dentists can make better diagnosis and provide better service to their customers.

Shown is the Vantage Panoramic X-ray system with the C-Arm that holds the lasers as well as the removable CCD receptor..

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This article was featured in Today's Medical Developments magazine and can be viewed at http://www.onlinetmd.com/TMD-0912-motion.aspx

Tags: Power-Core gears, vibration, PA12GC, calculating backlash, plastic gears, PA12C, PA12G, anti-backlash backlash gers, Medical

Moisture Absorption, Swelling in Nylons, except Cast Nylon Powercore

Posted by Simon Barrell on Nov 25, 2009 12:07:00 PM

Moisture absorption is a key factor to consider during nylon material selection. Design engineers need to take account of the application and tolerances because, machined component dimensions for a particular nylon grade will change if they were originally designed for a dry environment but are then used in a humid location. Moisture is absorbed by the part, and at some point when it reaches equilibrium, swelling can occur, and the fact that the dimensions of the part are subsequently altered increases the likelihood of premature part failure.

Injection molding, extrusion, and casting can all be used to form nylon, and the process used will influence material properties. In the case of injection molding, nylon is subjected to high compression and decompression within a short period of time. Correct mold design, timing, and precise pressure control are vital, and accurate thermal history for the part when both inside and outside of the mold should be maintained.

If water is present where extrusion is used in forming a part, severe problems can result. Therefore, moisture content and cooling rates should be monitored rigorously as they will both influence the physical properties of the extrusion.

Extruded rods or slabs can also be susceptible to shrinkage voids that are caused while the inside is in a molten state but the exterior surface begins to solidify. While the interior cools and shrinks under thermal contraction and crystallization, a void forms as the exterior no longer contracts. One solution is for extruders to use controlled water or vacuum quenching to cool formed parts. Note that even where forming processes can be precisely controlled, physical properties and dimensions may change over time when exposed to changing transport and storage conditions.  

Chart - Dimensional change polyamides versus moisture

The diagram shows moisture absorption causes swelling which results in up to 3.3% dimensional change in the nylon 6 family. When designing with nylon 6 or 6/6, this has to be taken into consideration, for example, when calculating backlash in gears, or when storing precision parts for prolonged periods of time in changing environments. For cast nylon powercore such change is negligible, even when permanently immersed in fuels, oils, and chemicals.

Tags: cast nylon 12, moisture absorption, calculating backlash, dimensional change, forming nylon, gear backlash, injection molding, material shrinkage, moisture content, polyamides