TTRPG Mechanical Trap Design - Simple Machines (Levers, Pulleys, and Inclined Planes) 101

June 30, 2020

Anyone who’s paid attention to my rantings about traps in TTRPGs should know that I think having some understanding of how a trap’s trigger(s) and effect(s) actually work is critical.  Without that information, the players’ potential to interact with the trap is undercut to the point of farce.  While many of the details may be Hand Waved for traps that work by magic, some more care is necessary for mechanical traps to make sense.

Admittedly, as someone who works as a mechanical engineer, I’ve got a skewed perspective on the topic.  Still, I don’t think it takes specialized education to be able to make use of basic mechanical elements in ways that lead to more immersive and creative experiences in actual play.  I’ve spent years running traps in this way for a variety of people, and it’s been a pretty rare experience for me to have a trap that feels boring.  I believe part of what helps me provide fun instances of traps is understanding how simple machines work well enough to provide immediate and sensible resolutions to actions that the players can process and react to.

Why simple machines?  They’ve been around long enough to work at any technological level, so there’s no need to worry about whether they “fit” in any context with intelligent tool-using life, as opposed to elements requiring special construction (e.g. springs, motors, gravity generators).  They’re common and straightforward enough for people to often have an intuitive understanding of how they work even if they don’t understand the details, so the GM can reasonably get away with describing what’s happening and leaving it up to the players to figure out why.  They’re capable of modeling a wide variety of triggers and effects, which means a little bit of learning how they work opens up countless possible applications.

There is some variety in what exactly counts as being a simple machine, but my basic definition is levers, pulleys, and inclined planes.  Wikipedia also cites wheel and axles, wedges, and screws, but the wheel and axle is something that wasn’t under the umbrella of simple machines in my public school education (it’s also kind of an oddball in that it isn’t really suited to redirecting force unless combined with other elements like gears or belts), and the latter two are specialized applications of inclined planes.

For this post, I’ll give an overview of each type of machine and talk about how it works, with a focus on qualitative generalities and core mechanics.  Depending on how I feel about it, I might expand this into a series with more detailed explanations and example applications, but I want this post to be more about the concepts rather than specific uses.  I’m going to be simplifying explanations, so please spare me pedantic comments about overlooking self-weight/friction losses/orthogonality/etc.

Levers

A lever is a relatively long object that has a point where it can pivot (called the fulcrum) and a force or load on either side of that pivot. Levers work on the basis of comparing moments (which I will define shortly) around the fulcrum.  If the moments from both sides are equal, the lever stays in place; otherwise, it moves in the direction of the greater moment.

One example of a lever is a crowbar, where the crowbar itself is the lever, the edge that it rocks against (or the contact point between a solid surface and the crowbar’s curved neck) is the fulcrum, your hands pushing/pulling on it are one force, and the lid/cover/etc. resisting the other end is the other force.  A seesaw or teeter-totter is another example, with the bar/beam itself as the lever, the rocking point as the fulcrum, and the people on either end as the applied loads.

A moment, sometimes also called a torque or a torsion, is a force multiplied by the distance from that force to a pivot point.  For instance, a person weighing 100 lbs. sitting at the end of a seesaw that extends 5 feet from the fulcrum is applying a moment of 500 ft-lbs.  This could be balanced out exactly by a 50 lbs. person sitting on the other end if that side extends 10 feet from the fulcrum.

Thus, the two key points to using levers:

 

1. A lever moves when the moment on one side is greater than the other.
2. The farther a load is from the fulcrum, the greater the moment that it creates.


A special case of levers (which I find quite handy for trap applications) is when one of the loads is an immobile mass, such as a floor, wall, or statue.  In this approach, light loads on the free side of the lever don’t do anything, but if a heavy load acts near the free end (or if the immobile mass is removed/destroyed), it triggers suddenly.  A classic example of this is the counterweighted hall trap in S1: Tomb of Horrors.

I find it’s easy to think of ways to use levers as trigger mechanisms, but they get also be great as part of the effect.  Releasing a held lever can be a simple way of launching dangerous objects (catapults can be thought of as a lever where the force on one side is the elasticity of the arm) or causing sudden massive impact (modern spring-loaded mousetraps are a perfect example of this).

Pulleys

A pulley is a shaft or axle (often called a block) over which a rope (or cord, cable, sinew, vine, or any other such supple object) is pulled taut.  Manufactured pulleys (like a snatch block or a block and tackle) usually have a grooved wheel for the rope to sit in(*), both to keep it in place and to make movement easier, but this isn’t strictly necessary.  Depending on how they’re set up, pulleys can redirect force (e.g. in the case of a rope slung over a tree branch, pulling down on one side causes an equal force to pull up on the opposite side), amplify force (e.g. with the same set-up, pinning one side of the rope in place and pulling on the other side will pull down on the branch with twice the force), or do both (e.g. looping a rope once around two branches, pinning on side, and pulling on the other side will pull up on the lower branch with twice the force and pull down on the upper branch with four times the force).

(*): In a more modern setting, replacing the rope with chain and using gears instead of wheels can also be a reasonable alternative, with the advantage of reducing slipping at the cost of being easier to foul.

The basic principle behind pulleys is that a taut rope has to have the same amount of tension in the entire length between its anchor points, so anything that it is pressed against or anchored to also experiences that same force.  One of the big tricks with this is that nothing stops the same rope from applying that force multiple times by taking advantage of loops.  Getting creative with rigging can lead to incredible force multiplication, limited by little more than just the total load that the pulley structure itself can withstand.

As with levers, using pulleys in traps often involves setting up a situation where things are kept in a careful balance until some outside action sets it in motion.  This generally takes the form of either a suspended object crashing down when the weight on the other end of the pulley is removed or two or more pulley blocks crashing together when a rope is pulled on suddenly (such as by a weighted end dropping off of its perch).

Compared to levers and inclined planes, pulleys can seem a bit awkward to use if you aren’t used to working with them in real life (I say this as someone who doesn’t work with pulleys much).  Their greatest strength is probably how much less space and time they need to work.  Spreading pulley blocks apart does little to nothing to improve performance (unlike levers and inclined planes, where increasing relevant lengths will improve performance), so you only need enough space for the blocks and rope, and a pulley can go from completely inactive to full strength just as fast as tugging on a rope (or in the reverse direction just as fast as cutting it).

Inclined Planes

An inclined plane is a flat surface that isn’t level with some reference surface (typically the ground in the case of a ramp, but it can be something else when dealing with more portable inclined planes like wedges or screws).  An inclined plane works by causing a force that isn’t either parallel or perpendicular to it to generate smaller forces in both of those directions.  Without going into the math of how this works (since it requires more complexity than just addition or multiplication), the end result is that it’s easier to move an object along an inclined plane than it is to move it directly, requiring less force for a shallower incline (at the cost of needing more parallel length to cover the same perpendicular distance).

The obvious use of this (moving something heavy to a higher elevation) isn’t really of great use in making triggers; the use of it for effects, often by converting a heavy object’s self-weight into horizontal movement, is straightforward enough to not need further elaboration.  Where things can get more interesting is with more creative uses, usually involved wedges or screws.

A wedge can be anything with two flat surfaces coming to an edge at an acute (less-than-square) angle.  Although a sufficiently large wedge can certainly be used as a ramp, what makes wedges special is that they’re typically used by pushing/striking or pulling on a side opposite to the edge.  Pushing/striking allows for far easier insertion than forcing in a blunt object because the force gets concentrated on that edge, which doesn’t give whatever the wedge is working against much area to exert resistance (if this sounds like how weapon edges cut through flesh, that’s because it’s the exact same principles at work).  Conversely, wedges tend to be relatively easy to remove by pulling because the shape tends to release from whatever it was driven into far more easily than a straight object would.

Thus, wedges tend to work great for piercing, lifting, prying, separating, and stabilizing (by squeezing together objects that want to fall apart) when they’re pushed in, and they’re also great at reversing those actions when pulled out.  Though exceptions are certainly possible, wedge-based triggers tend to get tripped by pulling the wedge out, but wedge-based effects can go in either direction.

Similarly, a screw is an inclined plane that has been wrapped around a shaft.  When the shaft is turned, the rotating force (which is perpendicular to the length of the shaft) generates a force along the length of the shaft.  Depending on the exact circumstances, this can either move the screw or move other objects along the length of the shaft (or potentially both, such as how drilling into wood both pulls the drill bit into the wood and expels wood dust/shavings from the entry point).  The lengthwise force can go in either direction, depending on both the direction of shaft rotation and the direction of the screw spiral; in fact, in my professional line of work, it’s not unusual to use screws with opposed spirals on either end for adjusting the lengths of metal rods since that makes it almost impossible for incidental forces on the rods to loosen the screws.

Honestly, screws are something that I rarely use, mostly because they feel too modern to fit into the sort of medieval fantasy I tend to run (which I know is largely a matter of personal bias, since screws as a concept are thousands of years old).  I like them for applications where I want a seal that can be breached and plugged repeatedly and a wedge wouldn’t work as well, and I also like using them for conveyor/elevator effects.  Otherwise, they just don’t tend to suit my personal tastes.

Conclusion

That may all seem either very simple or incredibly esoteric, but understanding these three mechanisms not only lets you add a lot of descriptive depth to how traps work, it can also help you enable your players in thinking of ways to handle traps other than a boring “roll to disarm / okay, it’s disabled”-exchange.  Levers can be jammed or held with compensating loads.  Pulleys can be jammed or have their blocks/shaft/axles broken.  Inclined planes can be pushed, pulled, slid, turned, or collapsed.  All of them can allow for weaponizing the mechanism against opponents (either as intended or in an improvised manner) instead of just being disabled.

If you want a bit of homework, here are a few things that can be done with each of the mechanisms:


Trigger types:

1. Pressure plate
2. Pit cover
3. Switch
4. Timer


Effects:

1. Deadfall
2. Spray
3. Projectile


See if you can think of ways to implement each of those with each of the three simple machines.

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