Sovella workbench V-5.0

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SOVELLA WORKBENCH V-5.0 Gear mechanism with vertical drive screws with square thread. Final ratio, variable from 50:1 to 100:1 using adjustable hand crank handle Two gas springs Type 14 or 20 , 400mm travel 150kg rate, total 300kg spring assist targets average load. Possible 125kg gas spring rate to target the middle of spring rate variance / travel distance. Required Watts with maximum load of 200kg. @ 15mm/s travel with no friction loss. 200 x 9.8 x .015 = 29.4 Watts. Estimated 50% to 60% efficiency with 50% efficiency 29.4 / .5 = 58.8 Watts. with 60% efficiency 29.4 / .6 = 49.0 Watts. Any entry that has a watt estimate without efficiency is not accurate or realistic. Another option for this mechanism would be a ball screw in place of the square thread drive, but this would require a locking device to prevent back drive. SPRING ASSIST The previous version V 2.0 Sovella workbench V-2.0 (update 12-22) with a torsion coil spring on the transverse shaft will not work because a spring with a 300kg rate in these dimensions has a maximum of only 3 revolutions. Depending on the ratio a drive screw mechanism has a transverse shaft that turns about 50 revolutions with 400mm travel. This leaves a gas spring as the best viable alternative. A 150kg gas spring can be sourced as a stocking item. Type 14 or type 20 from this list. http://indgassprings.com/pages/prod_c_size.html The best advantage of a gas spring is the low ratio between compressed force and extended force. All (compression) springs increase rate as compressed and decrease rate as extended. Gas springs change rate much less than mechanical springs. The typical average is a 1.3 ratio or compressed force / extended force. http://indgassprings.com/pages/tech_k_fact.html Compare this with a 300kg torsion coil spring measuring 11mm wire dia x 75mm OD x 1100mm length. The rate at 0.4 revolutions is 17.4 Nm and at 2.7 revolutions is 121.5 Nm. 124.5 / 17.4 = 6.9 ratio. The ratio for the Type 20 gas spring as sourced is 1.4. The calculated rate of a gas spring is at 5mm from full extension. So with this example of a 150kg gas spring the fully compressed rate would be 150 x 1.4 = 210kg. 17.4 Nm extended, 121.5 Nm compressed, 6.9 ratio, coil spring compared with 150kg extended, 210kg compressed, 1.4 ratio, gas spring Much more even pressure for a gas spring over the full 400mm travel than a comparable mechanical spring. This leads to another aspect of spring assist that can be adjusted by rate selection to optimize user power output. We have already adjusted the spring rate to meet the average load. The minimum estimated gross load is 100kg, the maximum is 500kg, so the optimum spring for all possible loads is 300kg. But what if we also consider the varying spring rate over the travel distance? Why not place the 300kg rate at the middle of the varying spring rate? The ratio between each end is 1.4, so there is a 1.2 ratio between each end and the middle. So if the optimum mid rate is 150kg then 150kg / 1.2 = 125kg min, 125kg x 1.4 = 175 kg. max. Given that a gas spring's rate is based on extended force we need a 125 kg rate gas spring. This may allow for a choice in the next smaller series of gas springs. So not only have we optimized user power, this also may yield a cost savings. OVER CENTER LOCK ON MANUAL DRIVE SHAFT This is for systems that have a small amount of back drive. Like a 10:1 worm. This will not work for systems with a lot of back drive like a ball screw drive shaft, which will require a passive lock possibly similar to an auto seat belt mechanism. DRIVE SHAFT DIAMETER. The 25mm OD 10mm ID transverse drive shaft will work with any system that has the majority of the mechanical advantage between this shaft and the load. Like this system with the 13:1 screw drive. On systems with the main advantage between this shaft and the manual crank, like a 10:1 worm drive, then a 32mm OD 12mm ID shaft may be better, especially with the longer version benches. LEG ROLLERS These are hardened steel with bronze oil impreg bushings. These may require a hardened steel strip attached to the surface of the leg where they roll. They have an eccentric axle adjustment in the 2 rear rollers to allow for production variance. MORE NUMBERS 400mm travel @ 15mm/s = 26.7 sec. target time duration 400mm travel @ 50% efficiency @ 40 Watts = X sec. 200 x 9.8 x .015 / X = 20 200 x 9.8 x .015 / 20 = X = 1.47 15mm/1.47s 400mm travel @ 15mm/1.47s = 39.2 seconds. 400mm travel @ 50% efficiency @ 40 Watts = 39.2 sec 12.5 sec (46%) > target time duration 400mm travel @ 60% efficiency @ 40 Watts = 32.7 sec 6 sec (22%) > target time duration