Design Process Spring/Fall 2019

Preliminary Steering Architecture 

Left: Steering architecture top-down view; right: side-vide of steering architecture past the fire wall. 

Connection Points

From the upright to the steering arm, there will be bolted connections to attach the two together rigidly. The tie rods will have rod-ends (spherical bearings) or clevises as the connection point depending on what is needed, same with the center link. The centrelink will be connected to the rack using either a rod end (and bolted) or using a set screw and jam nuts. Past the firewall, there will be a female spline rod to connect with the male spline end of the rack. Connected to the female spline will be a rod, connected to a u-joint (to allow for tilt adjustability of the steering wheel) along with a tilting mechanism similar to the one shown below. Lastly, there will be a male spline connecting steering wheel to the u-joint with a telescoping mechanism (will probably require a female spline with a slide fit and pins for adjustability). The column will be held up using a mounting jig with a collar and L bar with a shear pin (to allow for collapse). 

Components Past the Firewall 

The components past the firewall mainly consist of a steering column, shear pin and steering wheel, however, additional features will be constrained by interiors. 

Interiors requests: 

• telescoping (+/- 3 inches) 
• tilting (+/- 15 degrees) 

These functional requirements drove most of the design of steering past the firewall, the addition of a tilting mechanism (with u-joint), shear pins and an extra shaft was added. 

Fixtures

For the steering shaft, a two-piece collar should be used to hold together the majority of the shafts due to their superior holding power to set screws (distributed forces vs point forces). As well, the shaft material will not affect the holding power of collars whereas set screws should be harder than the shaft material so when it is properly tightened, the set screw should actually indent the shaft (Source).

Telescoping Mechanism (accurate as of 10/19/2019)

• The telescoping mechanism allows the linear distance of the steering wheel to be adjusted by the driver.
• This system adjusts incrementally. It is not infinitely adjusting.
  • Infinitely adjusting systems allow for the shaft to be adjusted to any distance within a given range. Incremental adjusting systems only allow the user to adjust to specific locations along the shaft (picture gym equipment).
  • An incrementally adjusting system was used instead of an infinitely adjusting system; infinitely adjusting systems require a large frictional force to allow the telescoping shaft to rotate without backlash (allow it to rotate at the exact same speed as the collapsible column).
• Parts:
  1. Telescoping Shaft
     
     • Purpose: to allow the driver to adjust the wheel forward and backward relative to them. The total telescoping range was set by interiors to be 6".
     • It is a 7" long 3/4" shaft rigidly attached to the quick release of the steering wheel on one side and nothing else on the other side. It sits inside the collapsible column.
     • A shaft with a splined end (NOT SPECCED YET) was used to make this.
       • A splined shaft was chosen specifically as it is transfers torque more effectively than the other partially-circular shaft options (more effectively than a keyed shaft and a D-shaft). Having the shaft only splined at one end allows the other end to fit inside the quick release.
       • The outer diameter was chosen to be 3/4" to allow for more affordable quick-release options. 3/4" is a common shaft diameter.
       • Note: Any non-circular shaft (i.e. a D-shaft, a keyed/ partially keyed shaft, a splined shaft, a hex shaft, etc.) could have been used, but the splined shaft was the best option for our application. Non-circular shafts minimize backlash.
       • The length is ____" (NOT SPECCED YET, should be >7" for sure) to allow part of the shaft to be supported by the collapsible column while it is extended to its limit (6"). The extension limit was set by the interiors team.
     • There are 12 x 3 mm OD holes in the telescoping shaft, each spaced 10 mm apart.
     • 12 holes and 10 mm are both arbitrary numbers.
     • The holes are 3 mm in diameter to allow the nose of the spring plunger to fit in them.
  2. Collapsible Column
     • Purpose: to hold the telescoping shaft, the spring plunger, and the spring plunger's mount in place.
     • It is a hollow, D-shaped , ___" long shaft, pin-connected to the telescoping shaft on the end closer to the steering wheel, and welded/rigidly attached to a 5/8" shaft on the end near the U-joint.
       • It is D-shaped to accommodate the telescoping shaft shape.
       • The outer diameter is D-shaped as well (i.e. there is one flat surface) to allow a mount for the spring plunger to be attached to it.
       • Its inner diameter fits the telescoping shaft's outer diameter exactly to minimize backlash.
       • Its wall thickness of 1/8" and its top surface width of 1/2" are both arbitrary.
     • It has a 6 mm OD hole on the flat surface, with its center being 20 mm away from the nearest edge.
       • The hole diameter is 6 mm to fit the M6 screw body of the spring plunger.
       • The distance of 20 mm is arbitrary.
  3. Spring Plunger
     • Purpose:
       1. Lock the axial and rotational motion of the telescoping shaft relative to the collapsible column while the car is in motion (i.e. while steering is happening).
       2. Allowing the user to easily adjust the telescoping shaft axially while the car is not in motion.
       3. Act as a shear pin in the event of a crash. The spring plunger would shear into 2 separate parts, allowing the telescoping shaft to collapse into the collapsible column, which would prevent the driver from being impaled.

• The current design uses 1 spring plunger (from here) which is initially loaded in compression. It fits into holes spaced ____mm apart. 
  • The hole spacing is arbitrary.
  • The hole is not conical since it does not significantly reduce backlash if the hole aids the spring plunger in centering itself within the hole. The main thing which aids against rotational backlash if the non-circular telescoping shaft.
• The collapsible column holds the telescoping shaft in place. The spring plunger is attached outside of it using a mount.
  • The mount is to be manufactured out of a small piece of metal.
  • The collapsible column is D-shaped (both inside and out). It may be manufactured via the following process:
    1. Make a hole axially through the shaft.
    2. From one side, mill the hole to have a D-shape. Repeat with the other side. This is done in 2 parts since the shaft may be too long to be milled in one step.
    3. Mill off the top surface to be flat.
    4. Tap a ___mm diameter hole through the side for the spring plunger to go through.
    5. Tap 4 more holes of __mm diameter for the mounting part to sit on.
    6. Screw on the mount with the plunger inside.

Tilt Mechanism - (Removed?)

• The tilt mechanism is used to adjust the angle of the upper column and steering wheel by raising upper column. It allows the upper column to rotate about the U-joint.
• The tilt mechanism is also used as a part of the steering column mounting.

• The guide plates (the side plates) are fixed in place. Spring plungers are used on the sides to allow the driver to incrementally adjust the height of the bearing housing plate in the middle. The same type of spring plungers are used for the telescoping mechanism.
  • Backlash may be an issue if spring plungers are used.
• The bearing in the middle of the mechanism holds the shaft in place, allowing it to rotate freely.

U-Joint

• The U-joint connects the lower steering column to the upper steering column. It is a part of the upper steering column assembly, even though it connects both the upper and lower assemblies.
  • It allows torque to be transferred from the steering wheel, but also allows for the steering column to not be one continuous shaft. this helps with packaging (space constraints within the car).
• It has an inner diameter of 5/8" and an outer diameter of 1 1/4" (from here).
  • The inner diameter dimension of 5/8" was determined by the lower column having an outer diameter of 5/8" to fit the 5/8-36 splined coupler which attaches to the rack.
• The U-joint was chosen to be made of steel in case it is to be welded to the upper/lower steering column.
• To mate this to the proper point of the master geometry, the feature Move/Copy was used in SolidWorks.
  • This was necessary as the U-joint CAD was supplied from the McMaster-Carr website as one part instead of as an assembly. Using this tool allows the part to rotate its individual components about a point.

Lower Column (accurate as of 9/21/2019)

• The lower steering column is responsible for transferring the rotation from the steering wheel to the pinion gear of the rack and pinion assembly.
  • It has the same function as the upper column assembly.
  • A lower column is required (instead of one long steering column) as the steering column must fit within the packaging (the space available in the car). I.e. the steering can NOT be like that of a dragster car.
  • It does not require a shear pin (for collapsibility: the upper column requires this to collapse in the event of a collision).
• The lower column is made of a 5/8" OD steel tube.
  • The outer diameter was determined by the need to join the lower steering column to the splined coupler which attaches to the spline on the rack.
  • The material is steel as the lower column likely will be welded to the splined coupler, and potentially to the U-joint.
  • A tube is being used instead of a solid shaft since steel is a strong material with a high shear modulus; for the required outside diameter of 5/8", a solid steel tube would add unnecessary weight without increasing the strength of the part by a significant amount.

Splined Coupler from Lower Steering Column to Rack (accurate as of 9/21/2019)

• A 5/8 - 36 spline is required to mate the lower steering column to the pinion gear of the rack and pinion (from here).
  • 5/8 signifies the outer diameter (in inches), 36 signifies the number of teeth
  • One of the sides is splined and the other is smooth (i.e. accomodates for a round shaft).
• The Pro-Werks 5/8-36 splined coupler was chosen (off-the-shelf part) because the Stiletto 340 rack from Pro-Werks was chosen.
  • This part would be unrealistic for us to manufacture ourselves based on our level of expertise.
  • Moreover, the exact dimensions of the teeth were not provided; the female coupler would be impossible to manufacture accurately without these dimensions.
• The coupler is to be welded to the lower steering column. Thus, the lower column should be made of steel instead of aluminum.

Stiletto 340 Rack

• The Stiletto 340 rack was chosen since other Solar car teams at FSGP 2019 used and recommended the same rack (from here).
• It has a rack travel of 4 5/8", a length of 11 1/4" between each rod end hole center, 3/8" rod ends, a 12:1 steering ratio, is 2.34 lbs, and comes with a 5/8-36 splined coupler.
• Most dimensions in the CAD were estimated and are not accurate; we called Pro-Werks sometime between June and July 2019 for additional dimensions, which they did not disclose.
  • The splined end of the rack had an outer diameter of 5/8", however, the exact tooth profile is unknown. Thus, the CAD for the spline is not as accurate as possible, but it would only matter if FEA is required for the rack. It is assumed that the rack will withstand the loads required for steering the car.
  • the pinion and rack tooth profile is not accurate for the same reason.
  • Dimensions were estimated by scaling actual images of the rack (provided on the website) to the appropriate size on AutoCAD, and extrapolating dimensions from there.

Notes from end of August 2019

Current CAD Assem (August 27, 2019)

Component Materials

• Steering columns will have to use splined stock to maintain rotation between the wheel and all components when the steering column collapses (and to connect to U-joint). 
• All shafts below the rack will have to be threaded to connect to components since they need to be threaded, the shafts should be made of steel. 
• All other components in the system can be made from aluminum. 
• Probably going with the nylon shear pins we had on MSXII, unless better alternatives can be found

Plan Going Forward 

DRI Document Fall 2019 

Resources

Steering CAD Remodel September 2019 (Google doc)