Petra
A belaying, rapelling and ascending device with an innovative safety feature
I developed Petra as part of a team of 22 for the MIT mechanical engineering capstone class 2.009 - The Product Engineering Process, and was one of two system integrators responsible for managing the project overall. I was also a technical lead.
Petra is a belaying and rappelling device, similar to a sticht plate (ATC), but with an innovative safety feature. In the event of the end of the rope passing through during a controlled descent, Petra will automatically lock on to the end of the rope and arrest a fall.
Challenge
Our brief for this project was to come up with an idea for a new product, develop it over a 6 week period, before presenting an alpha prototype in a product launch to an audience of 1100, complete with business plan and a live demonstration. After an ideation process where team members presented different product concepts, we settled on an auto-arresting climbing device.
We then opened up the design space again and considered three different concepts. We also set up subteams to look into different fabrication techniques and to collect user data from climbers and cavers at a local climbing wall.
I was responsible for coordinating these teams: by nominating a ‘team leader’ from each subteam, I could organise small-group meetings and effectively stay abreast of developments. In addition, I worked closely with the other system integrator to create detailed agendas for our weekly full-team meetings, to ensure they were productive and ran smoothly.
How It Works
All three concepts work using a similar principle: when the rope passes through both sides of the device, it can move freely in both directions; the climber controls the angle that the rope leaves the device to change the friction force on the rope and so can change the speed of descent. However, if the free end of the rope passes through, a cam with a spiral profile moves into the rope and engage it. The spiral profile acts to create a positive feedback loop: the friction between the rope and the cam causes the cam to rotate more, which compresses the rope and increases the friction force.
The device also doubles as an ascending device: when the rope only passes through one side, the sprung cams will only let the rope pass through one way, just like a normal mechanical ascender.
Solution
Testing showed that the Single Cam design worked best, so this was refined and exhibited at our technical review session. The central cam was waterjetted out of 6061 aluminium to create the toothed profile, the rope guides were 3D printed out of ABS, the backplate was cut from box-section aluminium, and it was all assembled with bolts. This early prototype was capable of catching a 120lb weight dropped by 1 meter.
After our technical review, we had to refine the design and prepare for the final presentation. To meet our original product specification, we had to double the maximum load to 240lb. We did this by identifying some key areas for improvement:
- Decrease friction at the pivot - achieved by turning the pivot pin to +-0.002” tolerance, and reaming the hole in the cam. Silicon-bronze washers were also used.
- Decrease friction between roller and rope - in order to let the rope travel smoothly when the cam was not engaged, we use an aluminium roller with bronze bushings.
- Reduce wear on the cam - our testing showed that the teeth on the aluminium cam were already dulling from repeated catches. We solved this by jetting the cam out of stainless steel instead.
- Improve useability - after showing the prototype to climbers, there was confusion as to how to load the rope into it correctly. We improved this by anodising the front plate, then laser-etching a graphic onto it to show the correct method.
- Improve ascending feature - by adding a hole to the top of the device, a small rope can be passed through and attached to the climber’s harness. This holds the device steady against their chest.
Presentation
The final step was to present Petra to a live audience of 1100 people, with 2500 more watching online. I helped field questions on the technical side of things in the Q&A session that followed the main presentation, which you can watch below. The presentation was a success: we successfully demonstrated Petra live on stage, as two climbers rappelled from the ceiling - one using Petra, the other using a traditional ATC device, with Petra’s superior safety features clearly highlighted against the traditional device. You can also find the brochure that accompanied the presentation at the link below.
