Ergorest Arm Support


Requested Project

Please refer to http://alimed.com and look at the ergonomic arm support that has the caption of smooth action.  I do not need to necessarily have that device, but one will serve the same purpose.  I need it to be able to clamp on a table and/or wheelchair tray.  I need it to be able to move in various directions, vertically and horizontally.  The support needs to be curved deep enough so that the consumer's arm does not slide off easily.  This is an important device for this consumer.  It will reduce her frustrations and be able to work independently.

Design Notes

Design Description:  The egrorest arm support shown in Figure 1 consists of a cradle to support the wrist to elbow area of the consumer?s arm supported by articulating arms attached by a clamp to the edge of a table or work surface.  All of the joints in the support arms allow rotary motion so a wide area can be traversed by the support cradle.  Please use the additional information button at the side of the project description to see pictures of two versions of the arm support that were delivered along with alternative mounting methods for tables or wheel chairs.

The cradle shown in Figure 2  is sawed from 4? plastic plumbing pipe and the edges are smoothed and contoured to eliminate sharp edges.  The cradle is mounted to a large washer by four #6-32 flat-head screws.  This washer is bent to conform to the contour of cradle.  It is brazed to a ?? OD, 3/8? ID steel tube that slips into a nylon bearing in the outermost support arm. The OD of the steel tube is machined to improve surface finish and provide clearance for free rotation in the nylon bearing.  A ?? shaft collar is used on this support tube to adjust vertical height of the cradle.

The support arms are made of ?? OD, 5/8? ID steel tubing.  A short piece of the same tube is used at the end of the arms to create a bearing housing.  The support arm is mitered and brazed to this short perpendicular housing similar to the construction of bicycle frames.  

The cradle support bearing consists of two nylon flanged reducer bushings, one pressed into either end of the bearing housing. These bushings have a 5/8? OD and ?? ID and were obtained at a True Value hardware store.

The center bearing between the two support arms is shown in Figure 4.  It has a plastic bushing that is available in hardware stores for the intended use of supporting furniture casters within a steel tube furniture leg.  The center hole is 5/16? in diameter and the outer diameter is machined for a press fit in the 5/8? bearing housing.  The end flange is also machined to reduce the diameter.  The outer support arm mounts to this bearing via a yoke that is secured with a 5/16? bolt.   Comment:  The use of the yoke limits rotational travel of this joint.  An alternative without this limitation would be to use the same type of bearing housing on both support arms, then stacking the bearings on a common shoulder bolt.

Figure 3 shows the clamp that secures the arm rest to a work surface and provides the suspension bearing for the support arms.  The inner support arm has a bronze sleeve bearing pressed into the bearing housing.  The sleeve bearing has a 5/8? OD and ?? ID.  This bearing rides on a ?? shoulder bolt, 2? long.  This allows for some additional vertical height adjustment if necessary with the addition of a collar on the shoulder bolt (not shown).  The clamp is bent up from steel mending strap material, 1? wide and about 3/16? thick.  It consists of two members brazed together.  The lower section of the clamp is tapped for a 3/8? x 16 clamp bolt.  The bolt itself is machined from a 3/8? carriage bolt where the head of the bolt is machined to the shape of a ball.  The brass clamp plate is machined to accept this ball and the edge of the brass cup is peened over into a depression between the ball and the remainder of the bolt to secure it to the bolt.

Project Status:  The design that is described below is similar to a commercial product that was suggested as a model in the project request.  However in some of the application areas (such as where a table is attached directly to a wheel chair) there are space constraints in front and to the side of the consumer that do not accommodate the articulating arms used in this design.  Consequently other approaches will be investigated.

Design Alternative for Ergorest Arm Support

Figure 5 shows an alternative design for an ergorest arm support along with a mounting bracket that is intended to engage the hollow-tube arm rest support found in some wheel chairs.  In this design the combination of telescoping arms with a swivel mount allow the arm support cradle to go to any location within a circle whose center is the swivel mount.  The radius of this circle is about 9-1/2 in.  The arm support cradle can approach within about 1-1/2 in. of the center of this circle, although with some sacrifice in the outer radius, design modifications could be made to allow it to pass through the center if the circle.  The mechanism has a very smooth action with low friction.

The basic parts shown in Figure 5 include the arm rest cradle described in connection with Figure 2 above, a telescoping support arm composed of ball bearing drawer slides, a swivel bearing and mount assembly, and the wheel chair mounting bracket.

The arm rest cradle is attached to the outer end of the telescoping arms by a brass block having a 1/2 in. diameter hole to accept the stem of the arm rest cradle.  The brass block is provided with 2 tapped holes on either side to attach the telescoping arms with # 6-32 flat-head machine screws.  Each tapped hole is provided with a small countersink to accept the taper of the screw head and permit drawing the thin support arm tight to the block.  The brass block is ?? wide which establishes the separation between the telescoping arms.  Twin telescoping arms are used to provide torsional stiffness against twisting moments that may be applied by the arm rest cradle when it is in service.

The ball bearing drawer slides used as the telescoping arms are made by Rockler Woodworking and Hardware, Manufacturer?s # 30804.  They are describe as mini drawer slides, 7-3/16.  They were purchased from Woodcrafters in Des Moines, $4.59 for a package of two.

The swivel end of the telescoping arms are secured to a U-shaped bracket cut from a metallic electrical box cover and formed into the U shape.  Spacing between the sides of the U is critical to maintain parallelism between the pair of telescoping arms.  The telescoping arms come with  tubular punched holes that can be threaded for # 8-32 screws to secure the telescoping arms to the bracket.  Screw length must be short enough so as not to interfere with the sliding member that moves past the screws.

The swivel bearing with its mounting plates is shown in Figure 6.   This bearing is made from the swivel bearing of a furniture caster having a plate type mounting.  The roller of the caster is removed and the metal sides that support the caster are flattened to provide a three-cornered mounting base for the bearing.  Slits are cut between the center and outer corners of the caster mounting to allow it to be spread and flattened without distorting the bearing race that is stamped into the same piece.  The swivel bearing is mounted to the bracket supporting the telescoping arms by 4 # 6-32 flat head screws.  Screw length is critical to prevent the screw ends from distorting the outer section of the telescoping arms which are located near the holes.  

The mounting base of the swivel bearing is secured to its support by 3 # 8-32 flathead screws.  Two of the mounting holes in this base are those which once held the caster.  A third is drilled in the middle section which protrudes between these existing holes.  A countersink is provided around each mounting hole.

Although various mounting brackets could be provided to secure the ergo armrest to different platforms, the one shown in figure 7 is intended to engage the hollow-tube arm rest support found in some wheel chairs.  The hole is normally plugged with a plastic plug that must be extracted to use this tube for mounting.

The horizontal tube that is used as the mounting attachment point to the wheel chair has an inside diameter of about ? in.    To slip inside this tube a second tube having a 3/4 in. OD is selected.  This second tube has the outer end modified to make it expandable to secure a tight fit to the outer tube.  The expandable section can be viewed in Figure 7.  Four ? in. holes are drilled at 90 degrees to each other around the circumference of the tube at 1-1/2 in. from the end.  Then the tube is slit to join each of those holes with the end of the tube.  The four sections of the tube now flex easily and can be expanded to be tight to the mounting tube.  To provide the necessary expansion, a brass cylinder is machined with a 3-1/2 degree taper.  The cylinder is center-drilled and tapped for a ?-20 bolt.  Figure 7 shows this bolt complete with a wing-nut.  This bolt is tightened to easily draw the tapered section into the tube and expand the flexible end of the tube.

On the opposite end of the expandable tube a ? in. shoulder is machined on a ? x20 locknut.  The shoulder is inserted into the expandable tube and the joint brazed together.  The expandable tube can then be bolted to the main bracket with a 1/2x20 bolt, ? in. long.  This bolt is center-drilled with a 9/32 in. drill to provide a clearance hole for the ? in. bolt that is used to pull the cone into the expandable tube.

The main bracket is cut from an L-shaped mending brace 1-1/4 in. wide and about 5/32 in. thick.  The upper side of the mounting bracket is drilled and tapped to accept the # 8-32 mounting screws from the swivel bearing mount.

Three ? in. diameter holes are provided to permit mounting the expandable tube in any one of the three holes for the purpose of height adjustment.  About 1-1/2 in. of height adjustment is provided in this way.

When the expandable tube is secured to the mating tube on the wheel chair, it may be subjected to twisting forces from the arm support.  In order to prevent rotation the lower arm of the main bracket is fitted with a U-shaped bracket that engages a vertical post on the wheel chair and prevents any rotation of the main bracket.

RCRV designer is Russell Schuchmann, and was requested by Options of Linn County - Jeannie Sampica. The project was begun Dec 5, 2004 and completed Mar 20, 2006.



Fig 1 Arm support



Fig 2 Support



Fig 3 Clamp



Fig 4 Joint



Fig 5 Alternate Design



Fig 6 Swivel bearing



Fig 7 Expansion tube assembly


2021 RCRV