Mechanical Sub-system Backup

Description of Mechanical Components

The animation below shows an exploded view of the core system and includes battery, fiberglass spring assembly, servo, drive motor, and spring clutch among other components.  

Major design improvements from earlier iterations include the placement of the fiberglass spring which has been re-oriented to pass through the center of mass and now resides in between the sidewall plates. The sidewalls themselves are now much thinner and smaller, using .062" thick aluminum 6061 sheet instead of .25" thick Delrin and possessing nominal LxW of 4.25" x 2.125".  In addition, the distance between the sidewalls has been decreased to 1.75" from 2.00". The standoffs holding the chassis together serve a dual purpose of structural support and component mounting, allowing the fiberglass spring assembly, servo, and battery to be secured.

Fiberglass Spring

A thin fiberglass composite strip called GC-70-UL from Gordon Composites was selected as the mechanical energy storage device on the RooBot.   The figure below shows the fiberglass spring on the RooBot chassis with a flex sensor mounted to the outer face.

For the fall prototype, the parameters of the spring were configured to be .030” thick x 1.5” wide x 6” long.  These parameters were selected after extensive testing of 4” sample springs with varied thicknesses and widths. 

The following figures show the relationship between force and displacement for various spring thicknesses and widths at 4" in length.

Spring Clutch

The spring clutch is the centerpiece of the RooBot design.  It allows for the motion of the main drive motor to be decoupled from the motion of the fiberglass compression and release.  The clutch consists of an input, output, transmission extension coil spring, outer shell, inner shaft, and release coil spring and relies on tight tolerances between the input and output.

When charging the fiberglass, the spring clutch input is engaged to the output drum so that torque is transmitted.  This winds the steel cable around the drum portion of the output, causing the fiberglass to gain potential energy.

Principle System Scientist Ben Brown provided the spring clutch design and a sample for use in the design.

Stranded Rope

The device that mechanically links the bottom of the fiberglass spring to the spring clutch is a .024” diameter 7x19 thread steel nylon coated steel rope.  The rope is held in place by Nicopress stop sleeves and compression sleeves which are permanently crimped into the appropriate position.    

Sidewalls

The sidewalls constitute the primary support structures for the chassis of the RooBot.  Together, they provide mounting locations for all other subsystem components.  This includes fixed mounting points for elements such as the battery, motor, and fiberglass spring.  It also includes rotation-mounting points with cutouts for transmission elements such as the gear train and the spring clutch (via flanged rotary bearings.) Below is a drawing of Sidewall A and a drawing of Sidewall B.

Sidewall A

Sidewall B  

Bearings

Two bearings are used to support the critical center spring clutch element.  To ensure that the bearings would remain secure in the side walls, flanged stainless steel shielded bearings were used.  The exact model chosen also had a special extended inner ring feature that allowed for a component making contact with it to rotate well with the bearing and thus reduce friction.

Fasteners

4-40 screws and nuts were used throughout the design wherever possible.  For structural elements, steel fasteners were chosen while for electrical components, nylon counterparts were selected.

The only element that required a different screw mounting was the GM21 gear motor.  This configuration utilized M-2 screws on its front face mounting.

Standoffs

Aluminum standoffs (1.75” long, 4-40 double sided female tap) provide structure for the RooBot chassis, connecting the sidewalls together.  These standoffs are also strategically used to provide mounting points to other components in the system such as the fiberglass release servo, fiberglass spring mounting point, and steel cable guide.

In addition to the aluminum standoffs, smaller nylon standoffs were used to mount the PCB and electrical components to the main chassis.  Two standoffs are used: a .5” hex male to female 4-40 thread and a .25” circular thru-hole version.  

Gears

The gear train of the RooBot is the key element that allows the GM21 motor to exert enough torque on the spring clutch to wind the steel cable to displace the fiberglass.

 The gear train is a 5:1 reduction going from a 14 tooth aluminum spur gear to a 70 tooth aluminum spur gear.  In addition to the 5:1 reduction, there is a 28 tooth nylon idler gear for purpose of the rotary encoder. This is driven by the 70 tooth gear.

 All gears are secured to their shafts by a set screw that makes contact with a flat cut into each corresponding shaft.