What was the biggest challenge in developing the Multiverse? By far, it was the design of the slipjoint spring . Traditionally, the principle is: the tighter the spring, the better. However, I couldn't apply that to the Multiverse because it's designed to be opened with one hand, so the resistance needs to be minimal when opening and high when closing.
Reconciling this apparent contradiction would have required countless prototypes based on trial and error . However, through my career as a computational engineer, it quickly became clear to me that I had the perfect tool at my disposal to solve the problem much more effectively. The finite element method allows for the calculation of components made of steel or titanium, for example. If you'd like to learn more, feel free to watch my YouTube video on FEM : https://youtu.be/RRKT0VvgK0I
The calculation results include the stresses within the component, as well as the forces and torques this component exerts on other components when it is deformed. This was exactly what I needed, as it allowed me to precisely analyze the resistance of the blade when folding and unfolding.
The image above shows the stress distribution in the slipjoint spring, which primarily serves as a visual check, for example, to determine whether the stress is plausible. It also allows me to identify the maximum stress in the spring. With this knowledge, I was able to define the strength of the spring material for my OEM partner Reate . The strength had to be selected so that the spring always operates within the elastic range of the material. Otherwise, the spring would permanently deform during use and lose its tension.
This calculation was performed in all blade positions. In addition to the stress representation, I analyzed the
torque required to move the blade. I've described why torque is a better measure than spring force to describe the strength of a slipjoint in a
separate blog post :
https://shorturl.at/JaCkh
I performed this calculation and evaluation process on over 30 design variants to ensure the perfect "walk and talk" (blade movement). The interplay between the spring and the geometry of the blade root is crucial. I quickly realized that it would have been impossible to have 30 prototypes manufactured. It would have taken several years and would also have been financially unfeasible. In this post, I would like to show you the first, initial draft and the final version of the production knife.
We'll start with the final production version and then compare it with the very first draft. The following diagram shows the torque curve of the production knife. The x-axis shows the angle that the blade moves through when opening and closing, and the y-axis shows the torque required to overcome the spring tension. For better understanding, I've divided the curve into 5 steps . First, the knife is closed (0°) and moves through the black line while opening. The torque increases steeply up to around 160 Nmm (similar to a detent ball) and then remains almost the same until the knife is almost fully extended at 140°. The constant torque at a low level ensures comfortable opening with one hand. Shortly before the knife is fully opened, however, the torque drops rapidly and changes to the green curve until the maximum opening angle of 169° is reached. The knife is now fully extended. The torque drops quickly and becomes negative. The negative values mean that the direction of the torque is reversed. To put it simply, the blade is retracted and clicks tightly into place without any additional force being applied by the hand.
To close the knife, a torque of over 600 Nmm must be applied, which is almost
four times the torque required to open it . This means that the user has good holding power for the blade despite the slipjoint mechanism. Once the maximum torque has been overcome, it drops to a level of around 50 Nmm (
red curve ), which ensures a pleasant feeling when closing. It is even possible to let the knife snap directly into the handle if you push it a little faster when closing it. The
red line merges into the
blue line at around 25° when the torque changes sign. From this point on, the blade is automatically pulled into the handle and held in place. This characteristic makes it act like a
detent mechanism .
However, it was a long way to get to this characteristic curve. That's why I would like to show you the comparison between the characteristic curve of the first draft and the series production knife. In the following diagram you can see the torque curve of the series production knife from the first diagram in black and that of the first draft in orange . The torque of the first draft is overall at a much lower level. Firstly, the knife opens with less resistance, which was too unsafe in this case. Secondly, the maximum torque for closing is 200 Nmm. This meant that the knife was not held open strongly enough and was unsuitable for many cutting tasks. With the series production knife this value was tripled to around 600 Nmm and now offers sufficient safety.
I am incredibly happy to now be able to present this sophisticated slipjoint mechanism on my Multiverse and look forward to your feedback.
Best regards, David
