Problems associated with Twackbot from Heck.

Challenge: Build a machine that uses a 2wd drive train to provide robot rotation with skid steer and produce maximum hitting force without melting the drive motors.

Robot weight 220 lbs
Coefficient of friction on drive wheels .9 or 90% (on a good day)
Battery power Hawker SLA's 13 Ah.
Drive tire diameter 10. 5 inches
Robot body diameter (tooth to tooth) 44 inches.

Motors

Magmotor   Motor weight      Motor H.P   Motor Torque at stall              Max RPM    Stall current
S28-150           3.8 lbs           3.0              1970 oz in (10.76 Ft lbs)         6000            480 Amps

Magmotor   Motor weight      Motor H.P   Motor Torque at stall                Max RPM    Stall current
S28-400           6.9 lbs           4.5              3720 oz in (19.375 Ft lbs)         4900            570 Amps

Black Max  Motor weight      Motor H.P   Motor Torque at stall                Max RPM    Stall current
NPC            15.5 lbs             3.8              281 in lbs (23.416 Ft lbs)           3400            470 Amps
 
 



Ok now we have some motors to play with, lets plug some numbers and see where the smoke becomes a problem.

The math I use can be found on another one of my web pages so I'm not going to explain it again here.

To move a robot you need torque applied to the wheels. The torque applied to the drive axles divided by the radius of the tire produces your tractive force. The tractive force needs to exceed the weight of the robot at all times by a minimum factor of about 2 to one. To keep the current draw of the motors at about half stall current draw.  (That's actually pushing it a bit, more torque and less current draw is better) I like tractive force factors of 4 to 5 to one myself, for a reliable safe drive train.



Twackbot from heck...
The idea of the Twackbot design is to obtain as much rotational speed with the entire robot body as possible. This is one of the more difficult ways to create a spinning weapon and will always put a high load on the drive motors.
Lets make some power and hitting force with the robots body and mass

The shorty 3 inch  Magmotor.
Magmotor   Motor weight      Motor H.P   Motor Torque at stall               Max RPM    Stall current
S28-150           3.8 lbs           3.0              1970 oz in (10.76 Ft lbs)         6000            480 Amps

Motor torque 10.76 ft lbs

10.5 inch tire   Adjusted tire radius in feet 0.4375

One mag motor          gear reduction                   Torque produced    tire speed MPH      tractive force 10.76 ft lbs                  4 to 1                              21.52 ft lbs             46.875                   49.1 lbs

two mag motors          gear reduction                   Torque produced    tire speed MPH      tractive force 10.76 ft lbs                  4 to 1                              43.04 ft lbs             46.875                   98.37 lbs

four mag motors          gear reduction                   Torque produced    tire speed MPH      tractive force 10.76 ft lbs                  4 to 1                                86.08 ft lbs             46.875                   196.75 lbs

 Even with four of these motors it's not enough power the tractive force is too low to prevent over heating the motors



The long 3 inch Magmotor.
Magmotor   Motor weight      Motor H.P   Motor Torque at stall                Max RPM    Stall current
S28-400           6.9 lbs           4.5              3720 oz in (19.375 Ft lbs)         4900            570 Amps

One mag motor          gear reduction                   Torque produced    tire speed MPH      tractive force 19.375 ft lbs                  4 to 1                              77.5 ft lbs               38.28                  177.14 lbs

two mag motors          gear reduction                   Torque produced    tire speed MPH      tractive force  19.375 ft lbs                4 to 1                                155 ft lbs               38.28                    354.28 lbs

four mag motors          gear reduction                   Torque produced    tire speed MPH      tractive force  19.375 ft lbs                4 to 1                                310 ft lbs                38.28                   708.57 lbs

It looks like four 3 inch Magmotors S28-400 would be the best bet.

If  you take the robots weight of 220 lbs and divide it by the .9 % coefficient of friction of the tires that gives 198 lbs of traction available (on a good day)

Four motors with a 4 to 1 reduction and 10.5 inch diameter tires produces 708.57 lbs tractive force at stall.
708.57 lbs tractive force divided by 198 lbs of traction available gives 3.578 times the robots weight in power produced. The stall current of 570 amps divided by 3.578 gives 159.3 amps current draw. Still pretty high but probably do-able.

Weapon speed estimate.
There's a few ifs used here
If the motors are happy and pulling an pushing the weight along in skid steer with minimal tire scrub.
The robot should rotate at the drive axles speed 1225 RPM's
1225 RPM times a 44 inch body equals 53900 divide by 336 equals 160.416 MPH weapon tip speed.
Not bad with 220 lbs of weight behind that it should hit pretty hard.
Boy with this kind of current draw I'd be thinking you would need about four 16 AH Hawker Oddesey Battery's...


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