STICKY DIP SKIDS

Issues with the starting of roll beds in an Ecoat wet skid system.

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General:

The skids in an Ecoat system that are not cleaned can generate a considerable amount of build up on the roll bed rollers. This will cause a higher-than-normal start up torque on the motor that can cause the skid to “stick” in place. After the skid has been setting for a time, the ability to start the skid moving becomes more evident. One of the issues is the build up of the uncured Ecoat on the rollers allows the skid rail to settle into the roller. The contact area between the roller and skid is effectively a point contact. With the Ecoat build up this becomes a larger contact area.

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Results:

The problems related to an Ecoat system that does not include a skid cleaner is the roll bed rollers are coated with the drip off Ecoat. When this builds up the contact area increases. The Ecoat is essentially a paste glue which adheres the skid rails to the rollers. The torque required to initially start the skid to move increases dramatically.

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Remedies:

The first step is to adhere to a cleaning regiment which will reduce any build up on the rollers. This is the first and foremost effective remedy. The second is to increase the motor size to overcome the starting torque. The motor drives on the roll beds are typically a VFD with controls to include acceleration and deceleration of the drive motors.

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Effect:

Newtons Law of motion is very simple, an object at rest tends to stay at rest. To get any stationary body to move, a resultant force is needed. In the cause of a skid on an Ecoat covered roller, the typical starting torque of the motor is not enough to overcome the body at rest. What is usually done to get these skids moving is to force the skid with bars or levers. This impact is enough to get an initial movement and start the skid traveling.

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New concepts:

To get any object to move that has a stickage issue is a sudden impact on the object. This can generate enough impact load to break the object free enough to allow movement. The concept is simple, a hammers weight does not have enough force to drive a nail into a piece of wood. The impact of the inertia does this. In this case a coupling with some free play can generate enough force to “bump” the skid off the roller. Experience with box style ways driven by acme screws has the same issues. For a slide to sit over the weekend or for any extended length of time, the saddle will settle onto the box way. To get this saddle to move initially took a considerable amount of additional force. The drive motor was coupled to the acme screw by means of a spider style coupling. Over time the elastic member would break down and allow a certain amount of internal clearance. This clearance generated the additional “bump” needed to start these sticky saddles. Using this concept, a coupling designed to have a certain amount of play will be able to generate the additional force to get the skid to move.

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Description:

Taking the spider style coupling and folding it back into itself. The drive sprocket is attached to an outer circular sleeve. The motor shaft attached to an inner ring. Lugs are positioned on both the outer sleeve and the inner ring. Springs may be used between these lugs to generate a required space instead of the worn “spider”. The spider can also be used if there is enough clearance. Since the motor is basically run in one direction, the lugs will always have this space on motor startup. Motors run in both directions can utilize the springs on both sides. The effect is when the motor is given the start command, the initial rotation is without any dramatic load. Once the motor has some speed, no matter how small or short in time, the motor has a tremendous increase in power. The “bump” effect only generates and additional load. As an alternate, there can be springs added on both sides of the lugs to reduce the impact load. Since the motor has already started, this generated inertia may not be needed.