The forces required to properly tighten a locking nut or compression washer are not
that great! I believe you may be building with the wrong tools. To properly tighten down a locking nut you need to use a properly fitting #2 Phillips screw driver or 3/32" hex tool depending on the screw type, and on the other end a set of pliers, or better a 1/4" socket driver like this.
In addition, if you can not tighten the screws in the bracket mechanical assembly very tightly you may have problems with things coming loose. If you do not have much arm strength, I strongly recommend you use a thread locking compound such as locktite on the screws that do not have lock washers or nylon locking elements.
Jim Frye, the Robot Guyhttp://www.lynxmotion.com
I've always tried to do my best...