Increase Mechanical Advantage

[Amesbury]

This is my first post here for please forgive me if am doing something wrong.

I am looking for ideas on ways to increase the mechanical advance of a mechanism that operates a 3-phase medium voltage vertical-break switch.

Basically it works like this: At ground level, a person rotates a handle (i.e. 24" long pipe) counter-clockwise and parallel to the ground to open the switch blades. Currently this works, but the person operating the mechanism has to be 'very strong' so I would like to make this easier to operate. The following photos show what this looks like:

 

[drawoh]

Amesbury,

I cannot see your photos. Photobucket is being blocked at my site.

You can use a longer pipe.

You can attach a gear drive or some action to your switch. Note how any increase in leverage increases your switch travel. At time point, you lose the pipe and replace it with an actuator wheel. You will need some means of showing clearly when the switch is engaged, disengaged, or between positions. There could be some serious safety issues here.

You can use a smaller switch connected to a relay.

--
JHG

 

[TheTick]

longer handle

 

[dhengr]

Amesbury:
I assume that the counterclockwise motion is your standard motion for opening the switch. Looking down on your third photo, the enlarged view of the upper mechanism; assume the gray portion (the lever arm, the crank) is oriented at 12 o’clock looking down. When you start cranking on the lower pipe handle, even with a large force, you do very little to induce rotation in the light blue switch blade actuating pipe. If you rotated the entire gray mechanism counterclockwise, on the vert. pipe (above the dark blue U-joint) to about 10 o’clock and locked it back down on the vert. pipe, you will have an improved mechanical advantage in terms of rotating the light blue switch blade actuating pipe. Then the main actuating activity would take place btwn. 10 o’clock and 8 o’clock on the lever arm, the crank. Since the switch blades are spring loaded, there must be some sweet-spot where the switch blades open and close. Maybe the crank should be set to act btwn. 9:30 and 7:30 o’clock. Draw some free body diagrams of this mechanism to satisfy yourself, do some field testing to verify this. You could also use the four bolt holes which increase the length of the crank the most. In you photo you use the four bolt holes which minimize the crank arm length.

 

[Compositepro]

I agree with dhengr. It looks like the mechanism is put together like an erector set for standardized components designed to be used for many different geometries. This particular geometry just seems like it was not optimized correctly. A few adjustments should improve the mechanical leverage considerably. Increasing the leverage may cause a decrease in the range of travel of the switch blades, however.

 

[btrueblood]

Along with what dh said, it looks like the handle is only intended to move through 90 degrees of rotation? Would a longer crank arm on the switch shaft, moving through a 180 degree arc, improve the MA for the same distance of travel of the switch contacts?

 

[TheTick]

Bearings

 

[chicopee]

If this were a proposed design, I would negate it. Since it is in existence I would look at increasing the length of connecting arm to the rotating shaft.

 

[tbuelna]

The kinematics of the linkage at the starting position seem to provide very low mechanical efficiency for a given input force at the lever. The linkage looks like it is designed to be self-locking (over-centered) at the end of the gray horizontal crank arm. Is this your intent?

 

[3DDave]

It certainly looks like the design was optimized on making adjustments to minimize difficulties from installation errors and to make it adaptable to a wide range of applications. As suggested above, it also provides a direct reading of the position of the switch contact, which multi-turn (geared/worm) drives won't do. The direct reading is a certain safety consideration.

It's hard to make any improvements because any changes will decrease the adaptability and use higher cost components and make direct reading more difficult.

The mechanism requires a high load over a small distance to engage/disengage. Then it requires a large travel to prevent arcing.

The first part of travel is very high leverage from the rotating arm, though offset by the lower leverage of the second arm. The rest of the travel uses increasingly lower leverage of the rotating arm to increase the distance, balanced by increasing higher leverage on the secondary.

I would probably create a program to plot leverage vs displacement of the output so I could try various combinations of lever lengths to see if there is any room for improvement. I am curious about the handle travel angle. It looks limited to 90 degrees, but I'm not sure if that's a firm constraint.

 

[Tmoose]

Is the effort to move the handle ~constant, and high effort?

Is the force to actuate the switch constant?
If not, as others said, modifying the linkage so there is high mechanical advantage in the portion of the travel where switch effort is high could lower overall effort to actuate, and maintain the existing handle travel.

 

[Amesbury]

Folks, I very much appreciate the responses.

drawoh & TheTick: I didn’t mention but I have already attempted a longer handle. Instead of a 24” long handle I used a 10’ long handle and was able to easily open it using one hand. I am sure you are aware that the trade-off with this is the increased travel distance by being further away from the pivot point (i.e. the vertical pipe). This is not really an acceptable solution. On the other hand, I will consider if gears are practical.

dhengr & Compositepro: Yes, although the mechanism is put together with standardized components designed to be used for many different geometries, I have free rein to change anything. Having said that, I can use a counter-clockwise or clockwise motion if I wish. What you described dhengr is exactly what I have experienced while testing this mechanism; the fact that it is very difficult to get the blue pipe moving with the grey crank at the 12 o’clock position. I am going to look into what you suggested. I would just like to point out the fact that these switch blades are not spring loaded. When opening the switch it’s a challenge to get the heavy blades to break loose from the stationary contacts and have them lift open against gravity. I hear what you are saying about possibly losing switch blade travel. When the switch is in the open position, it must be minimum 90 degrees from the closed position for safety reasons.

btrueblood: The handle can move more than 90 degrees depending on how it gets installed on the vertical pipe.

TheTick: Where would you suggest having bearings?

chicopee: This design is old and there is virtually no background information about it. Along with some other information I’ve received here, I will see what the effects are for differing lengths for connecting arm.

tbuelna: Good eye. It is ‘locked’ in the over-center position when the switch is in the closed position, and yes it is intended to be that way, however I’m trying to not focus on that aspect of the design right now.

3DDave: You are correct. Typically the people who install these mechanisms are less than attentive and sometimes do not even read the installation instructions and/or drawings.

Disregarding adaptability and cost, what kind of components where you thinking?

There is a mechanism on each switch pole that extinguishes the arc when the switch opens and there is a minimum speed for which the blade must engage the mechanism so that there isn’t a flashover.

I understand what you are saying and thank you for your input. No the handle travel angle is not limited to 90 degrees.

Tmoose: The effort to move the handle is not constant. Once the switch blades disengage from the stationary contacts, there is less effort required to complete the remainder of the opening travel.

 

[gruntguru]

So you need to reduce the input effort during the initial phase at the expense of increased effort for the remainder of the operation. It looks like you have 90* total rotation of the input shaft. Hard to judge but I would guess the output shaft also rotates about 90*??

The input bellcrank is correctly oriented to minimise initial effort (parallel to pullrod). The second bellcrank needs to be at 90* to the pullrod to minimise initial effort. Try increasing this angle (currently 30* - 40*) by rotating secondary bellcrank from the shaft and shortening the pullrod.

The Hookes joint (uni-joint) is another opportunity for introducing progressive MA. The top one is oriented correctly as shown in pic 3. Make sure the lower one is the same (with the switch closed, the cross should be articulated in the input yoke and not in the output yoke.)

je suis charlie

 

[Amesbury]

gruntguru: Currently the input shaft rotates about 115 degrees and the output is at least 90, possibly a bit more. I am planning on experimenting with the crank angle you mentioned. Thanks. Also, very interesting point you brought up about u-joint orientation. I was not aware that its orientation had an effect. Thank you for sharing that; I going to further look into it.

 

[TheTick]

Looks like you're using a U-clamp to hold up a member that must rotate about its axis. Lots of friction. Doesn't actually increase mechanical advantage, but would increase efficiency/reduce losses.

 

[Amesbury]

TheTick: If you are referring to the u-clamp near the handle; that's simply for connecting a grounding braid which just rotates with the vertical pipe.

 

[rb1957]

I don't see how the mechanism in the 3rd pic converts rotation of the vertical shaft (the 2nd pic shows the handle in the stowed position, to operate it is moved to the horizontal plane and rotates the vertical shaft, yes?) into rotation of the horizontal shaft, shown in the 2nd pic ? I'd've thought (probably wrongly) that the vertical leg would be moved vertically to cause the horizontal shaft to rotate ? (certainly that's the sense I get from the 1st pic)

another day in paradise, or is paradise one day closer ?

 

[IFRs]

While a 10 foot handle is too long, just doubling the current length will cut the force required in half, and a "very strong" effort becomes just a "strong" effort. The handle can be left as it is but supply the field workers with an extension that slips inside or outside the existing stub. This also tends to discourage idiots from messing with your equipment - if it is "hard" then more "thought" is required.