P11215detailed Design Review Pre-Readanthony Poli

P11215Detailed Design Review Pre-ReadAnthony Poli

Remaking of the drive system:

Goals:

·  Lessen vibrations induced while moving

·  Provide visibility to the drivetrain

Plan:

·  Increase wheel diameters

·  Replace hard wheels with pneumatic tires

·  Remove transport casters and put a clutch in the drivetrain

·  Fabricate chain guards out of polycarbonate or acrylic.

Why:

Increase wheel diameters:

Increased wheel diameters allow for more of the wheel to be in contact with the ground. This means that the robot will roll over smaller bumps and gaps, such as those created by the pavers, without shaking as much.

Replace hard wheels with pneumatic tires:

Pneumatic tires absorb shock better than the hard rubber tires that are currently on the robot. This means when a bump is encountered the wheels will absorb some of the shock, transporting less of it to the frame.

Remove transport casters and put a clutch in the drivetrain

The transport casters were small and created a lot of vibrations when being rolled around. By adding a clutch the larger drive wheels can be used for transport. This will provide a smoother overall ride for the robot and also make for easier conversion to transport mode.

Fabricate chain guards out of polycarbonate or acrylic.

New chain guards will be constructed out of a clear material to provide a barrier to the drivetrain. This will allow the drivetrain to be visible while still providing the necessary protection from the moving parts.

How it’s being done:

Rear Casters:

The shortest overall caster with a pneumatic tire comes in with an overall mounting height of 7.5 inches. This means that our robot is going to be higher off of the ground.

A new mounting plate also needed to be designed because new caster has a larger bolt pattern and the new front wheel will have the robot at an overall height of 8.10 in. from the base of the robot to the ground. No bolt holes are in yet because the piece is to be drilled to the caster for accuracy.

New Front Wheels:

The new front wheels are 12 inch wheelbarrow wheels. They have a foam core so that the tire never goes flat and strands the robot. We are buying one with an offset hub to make it easier to attach our drive cog to. The wheel comes with the rim, a tire and the bearings all assembled in one package for $34.07 each. (McMaster Part # 2336T21)

New Axle:

With the new diameter wheels the axle location needed to be moved forward. So the old axle is to be ground off of the frame and replaced with a new axle connected by two, bolt-on shaft mounts. The new axle will be ¾” in diameter because that is the smallest bearing size that comes with the wheels and the old axle is unusable. The new shaft mounts will also be easy to attach to and remove from the frame promoting a more modular design.

The Clutch / Transmission:

The new clutch is going to be a fabricated mechanical clutch created from purchased gears. The system will also serve as the step down transmission between the motor and the drive wheels.

The clutch will consist of two gears that move independently on a shaft, each with a hole drilled in them. In one hole the barrel of locking quick-release pin will be welded and in the other a smaller hole to fit the shaft of the pin. When the pin is in the out position the drive wheels will be free to move and when the pin is engaged the gears will spin together transmitting the drive force to the wheels.

When deciding on this system electric clutches were considered. They turned out to be too expensive when spec’d to handle the peak loads that would be seen by the system. This pin clutch system was considered after decent effort went into trying to obtain some electric clutches.

The gears are going to be a 72 tooth gear with a hub to help support the pin barrel and the other is going to be a flat 24 tooth gear to help save space. This gearing yields a 3:1 step down which is a little more than the old transmission, but with the new bigger tires the robot is going to move a little faster. All of the gears work with standard ANSI size 35 roller chain (3/8” pitch) which is the same that was used by previous groups.

The pin is a spring-loaded quick-release plunger style that has the ability to lock open by turning the know 90 degrees when the pin is pulled out. To re-engage the drivetrain, pull the pin out, turn it 90 degrees, release, and push the robot until the pin pops into place. Alternatively, the motor can run and the pin should pop into place.

There are slotted holes on the mounting plate to allow for chain tensioning.

Chain Guards:

The chain guards are in place to protect people from getting their fingers, hair, clothing, etc. stuck in the drivetrain. They are to be fabrication out of clear materials to provide visibility to working parts and show off the “mechanical beauty” that is inherit to the system.

The goal of the chain guard is to not allow anything near the gears. That means covering the system as much as possible. The current design is an odd shaped polygon that covers most of the gears from the sides. A bottom plate is in the works and a means to easily remove the front cover to perform maintenance is necessary and also in the works.

The holes in the outside face (the one in the back) are slotted to allow for chain tensioning. The one for the clutch handle is fine, but there needs to be more protection around the motor hole. This may be done by using a rubber shroud.

Bill of Materials: