If you’re staring at a pile of VEX Robotics parts or a digital assembly in Autodesk Inventor, you probably know the frustration of Project Lead The Way (PLTW) Activity 1.1.6. It’s that specific point in the Principles of Engineering (POE) curriculum where things stop being about simple levers and start getting messy. You're looking for the 1.1.6 compound machine design answer key because, frankly, calculating total mechanical advantage across three different mechanisms is a recipe for a headache.
Physics doesn't care about your feelings. It only cares about ratios.
Most people think a compound machine is just "two things put together." That’s too simple. In reality, it’s a series of force multipliers where the output of one becomes the input of the next. If you mess up the first calculation, the rest of your math is essentially garbage. It’s a domino effect of errors.
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The Reality of the 1.1.6 Compound Machine Design
Let’s be real for a second. The reason you can’t find a single, definitive "answer key" that fits every scenario is that PLTW intentionally makes this project open-ended. You aren’t just solving a math problem on a worksheet; you’re designing a system that must meet a specific Total Mechanical Advantage (TMA).
In the standard 1.1.6 brief, you're usually tasked with designing a machine that includes at least three simple machines. Most students gravitate toward a combination of a wheel and axle, a pulley system, and maybe an inclined plane or a gear train.
The "answer" isn't a single number. It’s the process. If your goal is a TMA of 5.0, and your first gear ratio is 2:1, your next two machines only need to provide a combined multiplier of 2.5. It's like building a puzzle where you get to choose the shape of the pieces, but the final picture has to be a specific size.
Calculating Mechanical Advantage (The Part That Breaks Brains)
The biggest hurdle in the 1.1.6 compound machine design answer key is the difference between IMA (Ideal Mechanical Advantage) and AMA (Actual Mechanical Advantage).
IMA is the world of "what if." What if there was no friction? What if the string didn't stretch? What if the world was perfect? You calculate this using distances. For a wheel and axle, it’s the diameter of the wheel divided by the diameter of the axle. For a pulley, it’s basically just counting the number of strands supporting the load.
$$IMA = \frac{D_{effort}}{D_{resistance}}$$
Then there’s AMA. This is the "oops, reality happened" version. You calculate this by measuring the actual forces using a spring scale.
$$AMA = \frac{F_{resistance}}{F_{effort}}$$
If your AMA is significantly lower than your IMA, your machine is "heavy." It’s inefficient. In the context of 1.1.6, the answer key you're actually looking for is the efficiency formula:
$$\text{Efficiency} = \left(\frac{AMA}{IMA}\right) \times 100$$
Honestly, if you get an efficiency over 80% in a high school lab setting using plastic gears and string, you're basically a wizard. Most real-world assemblies hover around 60% to 70% because of friction at the axles and the weight of the components themselves.
Why the Pulley System Usually Ruins Your Math
I've seen it a hundred times. A student builds a beautiful gear train, connects it to a winch (wheel and axle), and then runs a string to a pulley. They count three strings and assume the MA is 3.
Wait.
Is the last string being pulled upward or downward? If you’re pulling down to lift a weight up, that last strand is just a directional changer. It doesn't add to your mechanical advantage. This is the "gotcha" moment in the 1.1.6 compound machine design answer key. If you miscount your pulley strands, your entire IMA calculation is off, and your teacher is going to circle that number in red ink.
The Gear Train Trap
Gears are tempting. They look cool. They click. But they are also the primary source of friction in compound machines. If you use a 60-tooth gear driving a 12-tooth gear, you have a 1:5 ratio. That’s a speed increase, but a force decrease. For this project, you almost always want the opposite. You want the small gear driving the big gear.
- Simple Gear Train: Only the first and last gears matter for the ratio.
- Compound Gear Train: This is where the magic (and the math pain) happens. If you have two gears on the same shaft, you multiply the ratios of the two separate stages.
Specific Constraints of Activity 1.1.6
The PLTW rubric usually demands a few specific things that you can’t ignore if you want the "correct" answer:
- Three Simple Machines: You can't just use three sets of gears. That’s one type of machine used thrice. You need variety. A common "win" is a Lever (Type 1), a Wheel and Axle, and an Inclined Plane.
- Static State: The machine has to be able to hold a weight in place without it crashing down.
- Measurement: You must show your work for each individual machine's IMA before multiplying them for the total.
The formula for the total system is:
$$IMA_{total} = IMA_1 \times IMA_2 \times IMA_3$$
Don't add them. I know it's tempting. If you have an MA of 2, 3, and 4, your total isn't 9. It’s 24. Compound machines are incredibly powerful because of this geometric growth.
Common Errors in 1.1.6 Documentation
When you're writing up your design brief, there are "human errors" that make it obvious you didn't test the machine.
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First off, the "Effort Force" isn't just a number you guess. If you’re using a motor, you have to find the stall torque. If you're using your hand and a spring scale, you have to pull at a constant rate.
Second, the weight of the machine itself. Most students forget that the "Resistance" includes the weight of the pulley block or the lever arm if it's not balanced. If your lever arm is a heavy piece of VEX steel, it has its own mass that is fighting against you.
How to Build for a High Score
If you want to ace the 1.1.6 compound machine design, stop trying to make it complicated.
Build a simple inclined plane (the ramp). At the top of the ramp, place a wheel and axle (the winch). Connect that winch to a first-class lever.
Why this combo?
It’s easy to measure. You can measure the length of the ramp and its height with a ruler. You can measure the radius of the winch and the axle with a caliper. You can measure the lever arms easily. There are no hidden "directional" pulleys to confuse your math.
The "Hidden" Step: Friction Management
If you want your AMA to be anywhere near your IMA, you need to use spacers. In the VEX system, if you tighten a screw too much against a metal plate, the friction will eat 40% of your force. Leave a tiny bit of "slop" or wiggle room. Use Teflon washers if your kit has them.
Finalizing Your Answer Key
The true 1.1.6 compound machine design answer key is your engineering notebook.
Your teacher is looking for the "Work" section. They want to see:
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- A clear sketch (preferably an isometric view).
- Individual IMA calculations for each component.
- The multiplication of those IMAs for the total system.
- The recorded force from a spring scale to find the AMA.
- The final efficiency percentage.
If your efficiency is low, don't lie about it. Explain why it's low. "The friction in the gear mesh and the weight of the 2-hole cross-beam reduced our actual output." That is what an expert does. That is how you get the points.
To finish this project successfully, your next step is to perform a "dry run" of your measurements. Take your spring scale and pull your machine through its full range of motion. If the needle on the scale jumps around, your machine is catching on something. Smooth out the mechanical transitions before you record your final data for the 1.1.6 report. Once the motion is fluid, take three separate force readings and average them to ensure your AMA is as accurate as possible. This extra step proves you've moved beyond just "following an answer key" and are actually practicing engineering.
Next Steps for Mastery:
- Check your units: Ensure all measurements are in the same system (inches or millimeters) before calculating ratios.
- Isolate the friction: Test each of the three machines individually with a spring scale to see which one is the "weak link" in your efficiency.
- Refine the documentation: Cross-reference your final TMA with the project requirements to ensure you met the minimum multiplier requested by the rubric.