Many friends of us are closed to the circus world. Since we have mounted and dismounted circus tents many times, we realized how difficult it was to pull off the posts. Nox and I designed a tool that helps tent erectors to do this task. The principle is very straightforward : a long lever arm with a double pivot. The first pivot is used to transmits the force from the end of the lever to the post. The second pivot enables the device to catch the post and not to slip along it.
As the force required to pull off the posts can be very high according to the type of ground and the weather conditions, we wanted to build a device that would work in any situation. Thus we sized 'L'Arracheur de Pinces' so that it could pull off a post with a resisting force of 500 kgf in static (5000 N). This value has been calculated, based on the french safety standards for circus tents (standards 'CTS') : given that a post must resist to a force of 500kgf during the wrenching test (see below), we assumed that the force required to pull off the post according to the direction it has been stuck can't be higher this theoric value.
Since we wanted the weight of a normal person (about 75 kg) to be enough to pull off a post, we calculated that the lever arm should measure 2 meters long. In order to build easy-to-handle device, we reduced the lever arm length to 1200mm but an extension has been design to reach these 2 meters. The tube section of the lever arm has been calculated using these data with a safety coefficient of 2. The calculation has been performed with the freeware RdM6.
'L'Arracheur de pinces' is able to grip posts of different diameters thanks to its adjustable head. We drilled a groove in the piece called 'pavé' with 4 tapped holes so that the end piece called 'embout' could translate along it and be fastened at different positions.
'L'Arracheur de pinces'is entirely home-made. Each part has been cut, milled or turned by our hands (even the Wheels and the screw bolts !). It took one week to build the device. Currently, our friend Zouz, whose job is tent erector, often uses 'L'Arracheur de pinces' when it's too difficult to pull off the posts with a simple sledgehammer or even with a backhoe loader.