Slipjoints are folding knives in which the blade is held in place by spring tension, like most Swiss Army knives. Slipjoints are often discussed in connection with the spring force when opening or closing. However, specifying the spring force can be misleading. This is due to two properties from technical mechanics that we will address in this blog post. Because I was faced with precisely this problem when developing the Multiverse Slipjoint spring. In this blog post, I would like to point out the pitfalls in interpreting spring force and explain why torque is the better measure for indicating the strength of a slipjoint.
Firstly, this concerns the direction of the force to the axle bolt, or rather the angle at which the force acts. The second issue is the distance between the axle bolt and the position at which the spring force is specified or measured.
First, let's deal with the first point and look at how the force acts on the blade. I'll use the Multiverse as an example knife to illustrate this. The image below shows two directions in which the force acts on the blade to close it. Let's look at the right position first. The black arrow describes the direction of the force applied by the hand. The length describes the magnitude of the force. The black dashed line is the lever arm between the axle screw (rotation axis) and the force. If the force is not applied by the hand at a 90° angle to the lever arm, the forces split into two components (F1 and F2). In this case, the angle is 120°. Only force F2 actively works against the slipjoint spring because it is at 90° to the lever arm. The force component F1 has the direction of the lever arm and does not contribute to overcoming the slipjoint resistance.
How important the direction of force is becomes clear when we compare the left position. The magnitude of the force is identical. However, the blade geometry changes the angle to the lever arm, which is now 100° and thus significantly closer to 90°. This means that the distribution of the force components is less pronounced. Put simply, this phenomenon can mean that I can or cannot close the blade depending on the direction of the force. Conversely, it also means that I have to apply more or less force with my hand to close the blade depending on the direction. This is a major problem when comparing spring force specifications.
Up to this point, we've only changed the direction of the force but applied it at the same position. Another, even more serious problem is that the force changes significantly when the position of the force is changed. This is because, in a rotating system, torque is the correct descriptor, and force is more of an auxiliary parameter. While force is needed to calculate torque, the lever arm is always taken into account. The formula is M = F * L (torque = force * lever arm). The calculation example in the next figure will make this clearer.
The torque required to overcome the resistance of the slipjoint spring always remains the same. In this case, it is 600 Nmm. That's 0.6 Nm. However, if the position of the force application is changed, the lever arm to the axis of rotation changes from 43 mm to 82 mm. This means that the force required to fold the blade must change, as the torque remains the same. It is precisely this change in the required force that is reflected in the calculation result, as the change in position almost halves the force from 14 N to 7.3 N. The force specified in Newtons (N) can be converted into a weight for better understanding. 14 N corresponds to approximately 1.43 kg and 7.3 N corresponds to approximately 0.74 kg. The forces must now be converted into the actual hand force using the angle (16.2 N or 1.65 kg on the right and 7.4 N or 0.75 kg on the left). Here the effect of the first problem (direction of force) becomes apparent again because the deviation of the actually acting force F2 compared to the hand force is higher on the right because the angle deviates more from 90° there.
The two phenomena, force direction and force application position, make it impossible to compare slipjoint force specifications without precisely defining and maintaining the same force direction and position. However, the problem of force position can be circumvented by specifying the torque, because this remains the same regardless of where the force is applied.
In another article, I'll show you how I developed the Multiverse's slipjoint spring using the ideas from this blog post. I used modern simulation methods as a tool:
https://shorturl.at/dxwTs
Best regards, David
