High Tech High Top Hat Technicians. Gearbox Design as Seen Through the Toughbox. Gear Up

Transcription

1 High Tech High Top Hat Technicians Gearbox Design as Seen Through the Toughbox Or Gear Up

2 Toughbox Gear Pairs Diametral Pitch (DP): 20 per inch Pressure angle: 14.5 degrees Gear Teeth Outside Diameter (mm) Small Large N Pitch Diameter ( PD)= DP Pitch Diameter (mm) Small Large Sum of Radii (mm) (N 1+ N 2) Center Distance (C)= 2 DP Gears 14:50 16:48 19:45 24:40 28:35 N = 14 N = 50 Center Distance (mm) not to scale Gears Gear Ratio : Input:Output Ratio Small to Large Large to Small Not really a Toughbox gear pair Toughbox Overall Gear Ratios 16:48 19:45 24: :

3 Toughbox Motor Plate

4 Who Doesn't Like An Explosion? Toughbox Exploded View Bearings reduce the motor torque transmitted to the gearbox CIMs mount directly to the motor plate, requiring the plate to react the motor torque ½ inch output shaft and ⅜ inch input shaft

5 Gear Selection Transmission gears on AndyMark Diametral pitch of 20 per inch 14.5 degree pressure angle CIM gears 8 mm bore 2 mm keyway Cluster gear 9.58 mm hex hole Steel or aluminum Output gear mm hex hole Steel or aluminum

6 Thrust Bearings Thrust bearings allow two parts in contact and producing axial load to rotate relative to each other with little friction generated Thrust bearings work between two surfaces spinning at different speeds including one surface spinning and the other stationary Lower friction means reduced energy loss to waste heat Thrust Ball Roller Bearing Thrust bearings come in many varieties, but our designs will most likely use needle bearings or roller bearings Needle bearings High axial load bearing capability, but tend to wear unevenly Compact packaging Ball bearings wear well, but have less axial load capability than needle bearings When using needle bearings, consider using the mating washers as well to provide a smooth rolling surface Many online retailers sell thrust bearings, including Amazon Thrust Needle Roller Bearing

7 Design Process We need some additional requirements to narrow our design space and allow us to start designing the gearbox. We'll use these: CIM motors Efficient sustained speed of 3.0 m/s mm diameter wheels 20 diametral pitch gears Select our motor drive speed We will set our sustained speed as the normal load rotor speed of the CIM motor Using the table below, we select 4,320 rpm or rad/s as our design motor speed Load Torque (N-m) Speed (rpm) Current (amps) Power (watts) Efficiency (%) Free , Normal , Max efficiency , Max power , Stall

8 Design Process Calculate the rotational speed of our wheels when the robot is traveling at 3.5 m/s vtangential =wheel radius wheel angular velocity (rad / s) wheel angular velocity (rad /s)= v tangential wheel radius wheel angular velocity (rad /s)= 3.5 m /s m wheel angular velocity (rad /s)=22.96 Calculate the required gear ratio to match desired motor speed, rad/s, to desired wheel speed rad/s gear ratio= input speed output speed gear ratio= gear ratio=19.70

9 Design Process A speed reduction is fairly significant AndyMark gears max out at 14:50 (3.57 to 1) on a 9.58 mm hex shaft Two stages of reduction of 14:50 gears, as the Toughbox comes pre-configured, only yields an overall reduction of to 1 Let's see what we can do with three stages of reduction Three reduction stages Stage 1: 14 tooth, 8 mm CIM gear and 40 tooth, 9.58 mm hex; 2.86 to 1 reduction Stage 2: 14 tooth, 9.58 mm hex and 45 tooth, mm hex, 3.21 to 1 reduction Stage 3: 24 tooth, mm hex and 50 tooth, mm hex, 2.08 to 1 reduction Overall reduction is to 1 Calculate the center distances for each gear pairing 14:40 is mm center distance 14:45 is mm center distance 24:50 is mm center distance Center Distance (C)= (N 1+ N 2) 2 DP

10 Design Process In the Toughbox, AndyMark adds 0.05 mm to the center distance to provide clearance between the peak of the tooth and the trough of the mating gear With the center distances known, we can layout the gear shaft and CIM locations Check to make sure that gears and shafts do not clash between subsequent stages of reduction VexPro has drawings and STEP files of the CIM and min-cim motors available for download Mount points for the gearbox to the drive base should take into consideration the drive base design and locations of other drive and subsystem components

11 Considerations The gear reduction we need is a strong function of the linear speed capability that we want for the robot and the nominal speed we select for the motor. Play with the increase and decrease either or both and see how gear reduction changes As we increase gear reduction, we increase torque available on the output shaft. We will need to verify that mm steel shafts can handle the torque without failure or excessive flexure. We may elect to cantilever the motor off of the gearbox frame. Make sure sufficient material is available at the motor bolts to react motor torque and weight in motion without failure or excessive flexure. AndyMark uses a square polycarbonate plate for the motor mount on the Toughbox. More abstract motor mount designs are probably best executed in ¼ inch aluminum or similar. Have fun!

12 Questions?