limit switch

can the metal lever on a limit switch be bent, and still be considered legal for a competition?

Even if it’s bent, it’s still a legal VEX part.

Here is the reason I ask.

I previously asked if we could drill holes in the plastic of a motor for cooling purposes.

The answer was negative due to not being allowed to modify “electrical” parts from their “original configuration”

A limit switch is an “electrical/electronic” part. Without its metal lever it is “modified” (whether bent or broken off) and no longer in its “original configuration”.

I am glad we can still use broken limit switches, but I am really confused about the rule for modifying electronic (plastic) parts.

Thanks for the clarification.

We have bent the switch arm before and it has been accepted at Worlds. Be careful bending them, they will bend twice to three times before they snap.

If you use the pneumatic tubing over the metal arm you can extend the reach of the switch.

A limit switch is not an entirely electric part. The lever and button are both mechanical parts.

Modifying the motor by drilling holes in it is modifying the electrical component of the motor. Modifying a limit switch by snapping or bending the limit switch arm is modifying a mechanical component.

I don’t see how the rules are at all inconsistent here.

ok, then why did they say we cannot remove the backs of the motors where you can break the lever off a limit switch? Because if the lever is a mechanical part and the electronics is not being modified, the green backs should also be classed as mechanical because there is no electronics inside the actual piece of plastic.

Because anything you do to a motor might give you an unfair advantage over others. Once you change the rules to allow motor modifications you have opened the door to big-bucks teams playing games with the motors. If there is anything that will ruin the VRC competition, it would be rules that allowed modifications to motors. I think it’s safe to say that this will NEVER happen in VRC, and I don’t think there is any decision that would be less popular with the adult volunteers who have to figure out how to pay for the program.

Feel free to bend axle shafts, structural metal, or the little arms on the touch sensors. Just don’t mess with the motors or servos.

Removing the back of the motor affects how the electrical component’s heat dissipates. It’s not a mechanical mod.

As for the motor, the DC motor and the motor controller are the only electronics
The gears and the enclosure are both mechanical parts

Modifying the DC motor will be modifying the electronics, but modifying the enclosure will be modifying a mechanical part

I see an inconsistency with the rules

The electronics aren’t being modified
If you want to generate less heat through an electronics modification, you can simply kill a trace (ignore the fact that the motor won’t run anymore)

Just as a sidenote: What if you changed the “resistance” (probability of electrons transferred between two points; PWM, so not really resistance but an equivalent) between the power lead and the ground lead via an electrical signal introduced from outside the motor controller? That’d technically be a modification from the original state in an electrical sense… No teams are allowed to power motors :open_mouth: lol (this is inane, so no need to make a huge discussion about this part; common sense says no one cares)

I don’t. Modifying motors is not legal. Modifying some other parts is.

The enclosure very much affects the performance of the electrical part. The motor’s enclosure (as in the metal sleeve it’s in) has a noticeable effect on the motor’s speed if modified. Changing how heat dissipates will also affect the motor(electrical device)'s performance. If it didn’t, why would anyone want to do it?

my understanding was the modification would be done on the green/black plastic enclosure, considering how people have been mentioning “plastic” in their responses
it makes no sense to drill holes directly into the motor itself (i’d like to see someone accomplish that without hitting the coils inside)

also, changing the heat dissipation rate by making a legal modification isn’t against the rules
for instance, in the 3-wire motor, the motor controller is arguably sinked to the green plastic cover (no vents, so regardless of design intent, it’ll partially sink to the plastic)
if one were to have the green plastic cover in direct contact with a sheet of aluminum, the heat dissipation rate would change
would you claim that this is also against the rules?

the motor’s performance we’re dealing with would be divided into angular velocity and torque
angular velocity comes from the net torque applied to the shaft
this torque comes from current through the wire, producing a B-field
since the distance of the wire from the center is assumed to be constant (minimal expansion/contraction), the variables that remain are current and B-field strength
seldom is the strength of the B-field observed to change dramatically with ±2K change in temperature
as for the current, using I=V/R, voltage is a constant
which means resistance will govern the current flow
R=pl/A (should be rho)
length and cross-sectional area are just about constants, since the expansion/contraction isn’t of high magnitude
resistivity will change but only slightly
therefore the torque of the motor will not show too much of a performance difference (most everything is a constant at this level of temperature difference; get into the extremes and things will change)

modifications can also be made without gaining a performance advantage
for instance:
you harness a few cables together with a zip tie
the excess fits within the 18x18x18 cube and is not a source of entanglement
does cutting the excess of give you an advantage in performance?

Vex motors have thermal protection devices that shut down when hot.
All efforts to keep the motor cooler are most likely aimed at avoiding shutdown, (100% loss of performance) rather than gaining a few percent here and there.

Thermally isolating the thermal protection from the motor so that it doesn’t shut down will let the motor heat up even more, which is bad.

Keeping the entire motor cool by using aluminum heat sink and dedicating a motor and a polycarb fan blade to blow on the motors should be fine.

Precool the motors and electronics to 0 or -40C with an icecube or LN2?
Use of cryogenics should be discouraged to avoid the remote chance of accidently suffocating everyone in the room.
There would also be a condensation problem when mixing humid air with cold robot parts.

Optionally, teams could run their motors at maximum power instead of as close to stall as they can get. They’ll get an overall better robot that way without having to live on the edge of safe operation.

First off, the topic in discussion was “Legal vs Illegal Modification”
Mechanical modifications are legal, provided they follow the rest of the rules within the game manual; this point has yet to be argued

Second, venting will only marginally cool the motor
This won’t save it from hitting a critical temperature unless there is an active fluid motion around the motor in conjunction with the vents (convection alone would be insufficient when dealing with the extremes [aka when the thermal cutoff is set to hit])
There is also a physical power limit placed on the motors (P=VI,I^2R,V^2/R; take your pick)
Taking P=VI:
This is based on the voltage across the motor (approx. 8V for a fully charged battery pack with 100% power transmission)
“Max power” is achieved through an even higher current input
This current draw is dependent on the voltage across the motor
Unless the team uses a boost converter (or its like), the maximum will be at 100rpm or so
They’re all equivalents of eachother, so it doesn’t really matter which one you use
In essence, V, I, R are all constants

Now under a condition where the thermal cutoff is set to a lower temperature and the motors do shutoff, few other things must have happened prior to the shutdown

  1. The motor is stalled
  2. The motor is heating up
  3. Assuming this is in the driver controlled period, the driver isn’t doing anything about it (i.e. stop the motor)

If the thermal breakers hadn’t shut the motors down, the motors and/or motor controllers would have burned out eventually
In either case, they’d reach 0 power output (although one after the other)
This being within the rules, if a team is willing to kill their motors to get the few seconds of “advantage,” then so be it
It’s the equivalent of overcharging batteries
Battery life will decrease, and it’s not the safest thing to do in the world (depending on the degree)
It’s, however, not explicitly banned and grants an advantage
(although in this case, if “overcharge” can be defined and included as part of the inspection, it can easily be banned )

I trimmed down the quote and added reference numbers, but I don’t think I’m quoting your out of context.

  1. an electric motor is not a fixed resistor, for which P=VI is more useful.

  2. We are interested in maximizing the mechanical power output.
    We can’t increase the input current much, since the input currents are fixed by the H bridge and the thermal fusing circuits.

  3. The HS393 motor spec (corrected) shows that the motor current draw is dependent on the load, from 150mA no-load to 3.6A stalled load.

  4. I and therefore effective R are definitely not essentially constant.
    If they were, then we wouldn’t see a difference in thermal cutout if we change from low gear ratio to highspeed gear ratios, but we do.

V: Voltage delivered to the motor is relatively constant, but is reduced by IR factor of the supply circuits for high values of I.
I: The free current of an unloaded HS393 motor is 150mA, but the stall current is 3600mA, so I is not a constant, it depends on the load.
R: The DC resistance of the motor windings and R-on of the Hbridge is about R=V/I = 7v/3.6A about 2 ohms, when the motor is stalled. If the motor is rotating, there is a back-emf that resists current flow, effectively increasing the AC resistance. At a free current of 0.15A, the effective resistance is 7V/.15A = 46 ohms. So R varies inversely with I.

See also the other motor power thread:
“High strength motors and Gears” in the General forum,

total failure
thanks for the correction; totally forgot we were dealing with resistance of a motor despite the fact that we’re only dealing with motors
so with my “logic” a generator would “generate” no current… fun…

my point regarding drilling holes in the plastic enclosure being a mechanical modification still holds, though

From my understanding, this thread is arguing whether drilling holes in the plastic case is a legal modification or not. Simple answer: the Q&A said it isn’t legal, therefore, it isn’t. :slight_smile: What the Q&A says is “law”:

Emphasis mine.

Treat them like the Supreme Court: their ruling stands as the official interpretation, and will stand until they say otherwise.

You shouldnt have to make your motors cool down if you use them right.

Change how there used. Drilling holes in the back of them is asking for trouble eg Rust etc. Stupid idea.

they’re only talking about drilling into the plastic. but yes, there should be no need for it really…