please help screw jack lift

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dodgy

Joined Aug 25, 2018
1
hello everyone im new to this but we have fitted 4 post lift in small lorry 4/12volt motors 30amp fitted extra battery ,power not the problem .we cannot get the 4 motors to stay at the one speed any help would really be appreciated thanks at my wits ends dont know what to do
 

drc_567

Joined Dec 29, 2008
1,156
If the load, whatever it is, is not uniformly distributed between the 4 lift jacks, the load torque for one jack will not be the same as the others. It follows that different individual load torques will necessarily produce different lift rates. If you can install a rigid base platform, so that the load is shared more evenly between the group, then each individual jack will have more of an even load, and consequently there will be more equal lift rates. ... Suggest finding some type of rigid material ... Maybe steel plate ... something of that nature. This will not be an exact or precise solution, but may be adequate.
 

Glenn Holland

Joined Dec 26, 2014
703
My transit agency has corner lifts for buses and they are synchronized by encoders (that measure vertical travel) and the position is sent to a central controller.
 

dendad

Joined Feb 20, 2016
4,451
As above, you will need encoders on the motors. Then pulse width modulate the individual motor drives to keep them in sync. A processor control will be needed.
One added benefit will be you could control the speed and stop the motors id they get out of sync or have a motor fault.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
We built one for lifting rail box cars using 4 hydraulic jacks, we used encoders that had a steel cable that operated each encoder, the units were representative of a typical retractable measuring tape principle, where the wire extended out one way and retracted the other.
As mentioned you could put encoders on the motors, we used a Galil Motion card that has electronic gearing, IOW one of the motor encoders acts as a master and the other 3 gear off of it in a synchronized manner.
Max.
 

Reloadron

Joined Jan 15, 2015
7,501
While what you are looking to do is possible it is not simple. Most 4 pole lifts used for auto / small truck lifting generally have a single hydraulic ram cylinder being driven by a single motor which runs a pump. A series of cables are networked to the four points of lift.

Using 4 individual motors each motor will need to provide a feedback of travel to a 4 motor central controlled which is driven buy a computer. The motor controller needs to know exactly where each motor is at any given time regardless of any given weight on that motor pad.

Ron
 

drc_567

Joined Dec 29, 2008
1,156
The OP obviously delivers heavy cargo to various loading docks which require different elevations in order to roll off and roll on his cargo ... probably portable safes or something heavy. He would not require encoder precision, rather a means to control and equalize the angular velocity of the lift group, so that a reasonably level cargo platform is maintained at all times.
If the electric lifts are constructed with threaded screw mechanisms, then it should be possible to devise a way to create longitudinal magnetic channels or stripes on 4 quadrants of each lift screw. A Hall Effect sensor on each lift base would generate a digital pulse every time a magnetic channel traversed its pickup range. A digital counter would pause the current flow to each individual lift motor until an accumulation of 4 counts were received, one from each motor. A gate signal to a transistor or Mosfet would probably work. This technique would allow the cargo platform to remain reasonably level, within a quarter turn of the screw thread pitch, so that heavy rolling items would not get out of control.
A similar method of reversing the lift motor drive polarity would be necessary to cautiously lower the cargo platform.
Some form of solenoid controlled thread locking mechanism may be necessary to provide stable platform conditions when necessary.
 

Hymie

Joined Mar 30, 2018
1,277
To expect the OP to retrofit encoders and the control system required to control the 4 motors is not really a viable option IMHO. He would be best advised to design the system from scratch.

However I offer this solution which may work. Attach two (may need 4) tilt switches to the load bed in a horizontal plane at 90 degrees to each other – and use the output of these tilt switches to control the motors such that all the tilt switches are maintained in the level plane.

The control system would only need to know that the bed was ascending or descending and adjust the speed of one or two motors to maintain a level load. If each motor is controlled by a relay (30A) then the control system could be relatively simple (switching off/on motors as required).

I invite others to design such a simple circuit for the OP.
 

Hymie

Joined Mar 30, 2018
1,277
Here is a pictorial diagram of my proposal including the ‘truth table’ for the motor control, depending on the state of the tilt switches with the load bed ascending/descending.

Each of the 4 motors is positioned at the corners of the load bed, with the tilt switches located as shown.

In its simplest form the tilt switches need only be wired switching the relay coils of the relevant motors.

Once the tilt switches return within the level position, all motors will be powered.
 

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