Dampening Load Impact

Reduce the vertial travel to a minimum and control the descent onto the sample with a cam, ramp, damper or simply a slower moving linkage (think of the old record deck, arm lowering mechanism)
Reducing the cycles per minute may also be necessary to accomodate the slower mechanism, but without changing the mechanism it, won't change the load.
Conversely don't raise the stylus at all, just raise the load which is applying the force to the stylus. E.G Like holding a punch against the work piece and then striking the punch.
Sounds like a fun project, hope this helps.
Del

I appreciate the input. Reducing the vertical travel is a good idea, but to clear the samples, it was necessary to get a certain height. The samples are enclosed in acrylic boxes with 2 liquid injectors. The styli on the end are 1/2" rods that come through the top of the sample box. Since chemicals and different temperatures are used a cover was necessary. We are running at the reduced speed presently, but the force transducer didn't see a difference in impact loading until we got above 40 cycles per minute. I thought of having the styli at a fixed height and just adding the weight, but we are moving them biaxially. I couldn't come up with a rigid enough system to pull them sideways. I really appreciate your thoughts on this.

The overload is due to the speed at contact and the stiffness of parts. The first step would be to give an other speed profile to the stylus i.e. a fast displacement for lifting and for the approach stroke with an important speed reduction on the last travel before contact. The contact force would be controlled by the loop consisting of the force transducer and a controller of the actuator. This configuration allows a high cycling thus maintaining the test time low combined with a correct test since over load at start could bring unreliable results. The possibility to implement such a procedure depends on the actuator type you use and the overall system dynamics.

The unit is entirely mechanical. An eccentric bearing moves a crossbeam bar up and down. Thus the change in travel speeds would be very difficult to do. This unit is designed to give a consistent load for millions of cycles. We have had calibration problems with all of the spring or piston regulated machines we have tried. This unit was designed to use a dead load for a constant pressure all of the time for all stations and all cycles. Springs change over time and solenoids and pistons are hard to regulate. To install a full-time monitoring system for the 8 stations was cost prohibitive. I am using a load cell to calibrate the dead loads. It is working well except for the impact load. I am pretty well locked into the design that I have because we have put most of our funds (no-profit University) into the existing machine. I am looking for suggestions as to how we could reduce or blunt the impact. I don't think that a shock absorber in the system will work. The most logical thing to do would be to put a spring between the lifting bar and the stylus collar, but I am afraid that we might get bounce from this unless it is exactly designed. I was looking for anyone that could help me in the design of a spring large enough to blunt the impact, but small enough to not cause other problems. I would prefer any out-of-the-box methods that had not occurred to us. I appreciate all of your input on this.

The best would be ton send a sketch of your system since the way you designed it will more or less impose the solution. The spring if this is the only solution can only be designed considering the whole system as kinematics and the inertia of the stylus holder.

Belville washers look a good way of prividing the sort of spring action you need. You can series them or nested increase their stiffness.

I think your method is sinusoidal motion with a long connecting rod; could you drive with a pin into a radial slot with the shaft axes parallel but offset? (Between motor and 'crankshaft' to modulate the rotational speed).

Perhaps a search for quick return motions would help find your answer.

Maybe just putting a small leaf spring between stylus and operating lever/arm/whatever, something to absorb the intial impact, but such that it bottoms out and applies the full load a fraction of a second later.
E.G A stiffish leaf spring with say 3mm travel, it will have 3mm of slowly increasing force then it bottoms and applies the full load.
Leaf springs are good as they only have movement in one direction and are easy to mount.
Del
(Clamp 2 end to end and you have a bow)

Some interesting ideas. I will try the leaf spring idea and see what happens. It will be next week before the machine is open, and I can try some of these ideas. Thank you everyone.