# How hard is it to pull a goal?

I tried pulling the goals at kickoff @ Manchester, and those goals were really hard to pull… Even when it was just one goal. I had to set up my position, legs pointing forward, back bending backward, and really try hard to pull one…

THEN, I went back to GRT and visit, and saw a custom goal they built, and pull on that, and it was considerably easier… Then I started IMing people, and people who went to manchester kickoff said the goals were hard to to move around, and people who went to San Jose kickoff said the goals aren’t that hard to pull…

So, I am wondering if teams can do me (and to everyone) a favor, and use one of those spring scale, or a force meter, and figure out how much force it take to pull a goal. Two number would be nice: the force it take to get a 0 velocity goal moving, and the force it take to keep a goal moving…

Thanks!

There are several things that you need to consider to make a “goal pulling” experiment somewhat accurate.

First of all, the position of the casters is important when you start to move the goal. If the casters are in the correct position for movement in that particuliar direction, the force required to move it is considerabally less.

Secondly, you need to consider that for a human to push/pull a goal is different than a robot pulling or pushing a goal. While humans were designed to move forward, a robot could care less which direction it moves. (4-wheel drive tank drive train).

Just as a frame of referance, We did a few tests concerning the pushing/pulling of goals. We used our last year’s robot to try and push the goals.

Our last year’s robot was basically a 4 wheel trive tank system.

Drill motors were set at low gear for the experiment, and our gear ratio is 3:1.

We were able to move one goal at a pretty good speed.

We were able to move a goal with a 130 lb guy standing on it at a good speed as well.

We were able to move a goal with a 130 lb guy and 180 lb guy standing on it at a moderate speed.

Our wheel started to slip when we had a goal 130 lb guy 160 lb guy and 180 lb guy.

You can get a lot of stuff to move with a reasonable drive train.

pushing and pulling won’t be that hard. It is the turning aspect that will put the stress on the motors.

*Originally posted by Patrick Wang *
**
Drill motors were set at low gear for the experiment, and our gear ratio is 3:1.
**

If you don’t mind sharing, what was your wheel diameter with this set-up?

Thanks
Matt B.

Last year we purchased the specified casters and some “cheaper” ones. The goals made with the cheaper ones were CONSIDERABLY HARDER TO MOVE. In fact, after forgetting about the mixed casters, we decided that there would be lots of variation in goal resistance and decided to not try to drag goals around. Oops. In fact with the casters specified the goals are pretty easy to move around.

Well, let me qualify that: we used the “good” casters from last year. Perhaps this year FIRST spec’d the cheaper ones that Home Depot sells. I better check that out before we repeat last years mistake.

We too, use the Bosch in low gear with a final drive of 3:1. Plenty of power to slip wheels on the carpet. With gear switching, top speed is a respectable 10fps as well. (6" wheels turned down to 5.75" - flat)

Cheers!

Team #492

we are still waiting for our goal to be built…i’ll try to publish some results asap.

The goal isn’t as hard to move as most people (especially those on my team) think. When i was at the kickoff event in my area (Toronto) the goal moves along the floor with little to know effort. I’ll admit, when the wheels are turned in a different direction it will be a tad harder but not that much, as long as you use good quality wheels you should be fine.

speaking of turning, that is certainly something that we need to test. I’ll see if I can do that with our old robot tommorow.

did u guys notice that the ANGLE IRON was different at the kickoff goals and they are saying that is the RIGHT WAY, and the BLUE PRINTS are WRONG what do u guys think? O and BWT we “attached” our last years robot to 2 goals and the robot could make a wide trun with both on and a turn around itself (the center of the bot) with 2 people standing on it!

I can see one problem that could cause FIRST a headache. As the Goals are all placed in the same place to start the game and most Robots will aproach the Goals from the same direction, then I can see the carpet getting some what thread bare in those places especialy with the weight of the Goals and the need for torque.
Nick237

Why are you worried about moving the goal. the goal is the easy part.

Moving a goal when someone else is trying to pull it from you is going to be the hard part.

if you make a robot that is going to be just able to pull the goal you will be under powered

Moving the goals is pretty easy, but that is if noone is tugging on any of the goals. Attached is our robot from last year pushing 3 goals. Our latches from last year don’t work on these goals and we aren’t planning on destroying last years robot, that is why we are pushing it.

Some details:

Wheel diameter = 8"
Motor type = drill in low
gear ratio = 2:1
drive type = 4 wheel skid steer
tire material: Front - timing belt; Rear - Confidential

Disclaimers:

1. We did not modify last year’s robot at all.
2. We performed this experiment to get a data point on how hard it would be to move 3 goals
3. As you can see in the video, our speed was pretty good

The million dollar question: Will this be enough??? Time will tell.

NOTE: The voice at the end is mine. I didn’t know he was recording sound.

Well, I just got the message that I can’t upload the movie and I don’t want to delete everything. Does anyone know what I can do to upload the movie?

when we built our goaoland tested it with last years robot it tokk 80 pounds of pressure to move the goal.

At our remote kickoff, someone tied a cord around two of our batteres and ran the cord through a pulley to the goal. If the castors were prealigned, the weight of the two batteries would (barely) move the goal.

We measured the force (straight pull) it takes to move the goal on the carpet from a resting state using a fancy strain gage. It averaged out at around 32-35 lbs.

As stated a few times, but is worth stating again, turning at a decent speed while holding the goal (and while having decent traction against being pushed sidways) is the real design challenge.

The wonder wheels (wheel with side rollers offering side motion as well as the wheels rolling) worked pretty well last year for some, but this leaves you very vunerable to being pushed sideways.

It will be interesting to see what people come up with.

Scott358

As stated a few times, but is worth stating again, turning at a decent speed while holding the goal (and while having decent traction against being pushed sidways) is the real design challenge.

You need to think outside the box.

Turning if done right is very very easy. we can turn now with all three goal.

the fun part will be pulling other robots and still pull all three goals.

Watch the KSC in March 7-9

by using the above numbers-
the coeff of friction (for hard rubber vs carpet) for the goals on the carpet to be used in the comp
it is
Force of friction=mu * normal force

181= mu * 35 (assuming the max needed to move goal)

mu=35/181

mu (or the coeff of friction)=.19337

This sounds about right for the material in question

dahl
team 497
]

That equation doen not seem accurate

You put the weight down…not the force pushing down

Te acutal force on the casters is as follows

181 pounds = 82.1 Kilograms

so 82.1*9.8m/s^2 = 804.58 Newtons of force total

Now… to find the coefficient of friction…

Friction = MuMassGravity
or in this case ----> Mu(804.58)

so in order to move this goal…

You plug in the following

Force of Robots Push or Pull - MU(804.58) = 82.1* Acceleration

Now…to fin dthe minmum force needed to move the goal…

You set acceleration to zero…

So you get… Force of Robots Push or Pull - Mu(804.68) = 0

``````                Force of Ropbots Push or Pull = Mu(804.68)

Force of Robots Push or Pull / 804.68 = MU
``````

There you go…just plug in the force from your robot and you should find mu.

I need to check some notes…but there might be a better way…

And remember Its in Metric

G.Goldman

we usda s spring scale designed for bows and got somewhere around 28-30 lbs of force to get it moving when all 8 casters were pointing sideways.