Designing a Proper Trigger for your Pumpkin Launcher
Prof. Matthew Spenko
MMAE
[email protected]
Why a Trigger?
• You must remotely trigger your pumpkin launcher
• In some cases, the trigger mechanism may be carrying the entire load of the counterweight
• If you do not build a proper trigger, it will jam, causing massive headaches and wasting time
• This document will show you how to model a trigger mechanism so you can build one that works
Pin Trigger
• Probably the simplest type of mechanical trigger is a “pin” trigger
Pin Trigger Jam
• An improperly designed trigger can easily jam
• When this happens, it is extremely
difficult to fix and there is a time limit to launch your pumpkin Designing a Proper Trigger
• Draw a free body diagram on your trigger
• Sum forces and moments:
+ =
=
>
• Solve for
>2
+
+
:
An Example
• Let’s say you have a steel pin on a bronze housing (You can find some nice flanged sleeve bearings at McMaster Carr). The coefficient of friction of steel on bronze is somewhere around 0.3.
• Assume:
• You have a simple trebuchet with a counterweight of 50kg. The pivot point is equidistant from the counterweight to the trigger. That’s an FLoad of about 500N. • L2=3cm • L1=1cm.
• FTrigger = 400N! That is pretty large. Increasing L2 to 6 cm brings that down to 350N, but that is still a lot of force you have to pull with. • I am not sure how much a person can easily pull, but I suspect you would want something around 100 N. That is impossible with a pin trigger with this load! In fact, if you look at the equation, the lower limit on FTrigger is 2FLoad.
• To address this, you can:
• Lower the coefficient of friction, but you’re not going to get much better than 0.2.
• Lower FLoad. You can do this by adjusting the place where you place the trigger. Adjusting the Trigger Location
• By adjusting length from the pivot point to the counterweight, M and the trigger, you can adjust the load that the trigger sees. If LT > LCW, then you can decrease FTrigger to be less than the force caused by the counterweight
• If you designed a floating arm trebuchet or something else, draw the FBD to see the best location to put your trigger
Using Pre‐Fabricated Triggers
• The Physics department made a set of pin triggers a few years ago. They sometimes lend them out.
• Some competitors in the past have used them without doing the math to determine if they are appropriate for the particular design. Many of those competitors failed. • Don’t be a hacker, be an engineer and do the math!
Latch Triggers
• A latch trigger is a bit more complicated to build than a pin trigger.
• Keep it simple! If a pin trigger will work for you, then use it. • If you modeled a pin trigger and the math tells you that there is no way for you to get it to work, email me and I will put together some notes on a latch trigger.