Increase Mechanical Advantage
Increase Mechanical Advantage
(OP)
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:
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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:
[URL=http://s17.photobucket.com/user/escrayzee/media/Me...]
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RE: Increase Mechanical Advantage
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
RE: Increase Mechanical Advantage
RE: Increase Mechanical Advantage
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.
RE: Increase Mechanical Advantage
RE: Increase Mechanical Advantage
RE: Increase Mechanical Advantage
RE: Increase Mechanical Advantage
RE: Increase Mechanical Advantage
RE: Increase Mechanical Advantage
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.
RE: Increase Mechanical Advantage
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.
RE: Increase Mechanical Advantage
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.
RE: Increase Mechanical Advantage
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
RE: Increase Mechanical Advantage
RE: Increase Mechanical Advantage
RE: Increase Mechanical Advantage
RE: Increase Mechanical Advantage
another day in paradise, or is paradise one day closer ?
RE: Increase Mechanical Advantage