Maintaining Proper Exit Pitch of Your Containers
One of the most frequently asked questions customers have about timing screw conveyors (also known as feed screw conveyors) relates to the relationship between conveyor and screw speed.
Why are conveyor speeds and timing screw speeds different? Shouldn't they be the same?
The short answer is: NO. But, there's a science behind it.
For proper container handling with standard applications, conveyor speeds need to be, at a bare minimum, 10 to 15% faster than the rotation speed of the timing screw. The overall conveyor speed ultimately determines pitch because timing screws (feed screws) are designed to hold containers back on pitch while the conveyor speeds moves the containers out of the timing screw.
It can be complicated…we know, but other than timing, this is the biggest topic we discuss with maintenance and operations personnel each year, and the answer often allows for quick and easy troubleshooting that brings the equipment back to standard operating conditions.
Think of it this way: conveyor speeds pull the container out of the screw while the screw sets the pitch the conveyor will be pulling the containers out at. This means the proper set up speeds must be met in order to maintain accurate container handling when the containers exit the timing screw(s).
Unless a deadplate is involved, screws should not be designed to pull a container out on pitch but rather hold back the container on pitch. Why? If you push the container, it has some room to move within the screw pocket and you wouldn’t be able to guarantee consistent spacing of the container or that they are on pitch due to this wiggle room.
However, if you pull the containers with the conveyor speed, you know they are always riding on the leading edge of the timing screw which will regulate accurate positioning of the container.
It’s helpful to think about it this way: It is the conveyor’s job to move the container, but it is the timing screw’s job to handle the container.
We know, it’s still confusing unless you’re an expert, so follow along with the video examples below. These videos can also aid in troubleshooting timing issues if maintenance personnel and operators have changed the speed (it does happen) or you have a screw from a different manufacturer that just isn’t operating in time.
Before we go further, we need to define a few items and explain the basic math for understanding conveyor speed.
Required rate: containers per minute. (For our example, it is 100 cpm.)
Container axis: diameter of round container or length of rectangular/oval container. (For our example, it is 2.50”)
Timing screw discharge pitch: center to center of containers as they leave the timing screws. (For our example, it is 5.00”)
Based on a 2.50” diameter container, rate of 100 cpm and discharge pitch of 5.00”, the linear speed of the containers as they leave the screws is 500 inches per minute (100*5) or 41.67 feet per minute (500/12). This is the required speed for the conveyor.
The conveyor speed needs to be based on discharge pitch of the screw but we should also understand how conveyor speed can affect containers feeding into the screw.
Again, based on 2.50” diameter container and rate of 100 cpm, the conveyor must run at minimum of 250 inches per minute (100*2.5) or 20.84 feet per minute. If the conveyor runs slower than this speed, the timing screw will be starved of containers because they can not get there fast enough. At 250 inches per minute, this is half of the speed required for the discharge pitch (500 inches per minute).
So now we have established that our conveyor speed is twice as fast as required to feed the timing screw which may sound excessive. Because the conveyor is a low friction plastic construction, the containers do not fall over at the infeed of the screw. This low coefficient of friction allows the containers to remain stable as the containers accumulate at the infeed of the screw. (this can be seen in the video)
CORRECT TIMING | Proper Pitch:
In this example, the conveyor speed is 10% faster than the timing screw speed, and we are able to consistently and accurately maintain a 5-inch pitch between the centers of the containers. Because the screw lets go of the container and the conveyor pulls it out, the tall container is able to maintain stability on its base.
SLOW CONVEYOR SPEED:
With a slow container speed, you’ll notice the pitch is not accurate between the containers, and in fact it is too small. That is because the conveyor isn’t accurately able to pull the containers out of the screw.
In this condition, remember we are trying to achieve a 5" inch pitch as the containers are discharged. The conveyor speed in this instance is 36 fpm while the screw is 100 cpm. With 36 being approximately 86% of 41.7 (our ideal conveyor speed), the pitch in the timing screw is 86% of our ideal pitch. The pitch in this condition is 4.3" (or 86% of 5").
FAST CONVEYOR SPEED:
With a speed too fast, the containers are unstable and could fall over, and the pitch is now too large.
In this condition, remember we are trying to achieve a 5" inch pitch as the containers are discharged. The conveyor speed in this instance is 48.4 fpm while the screw is 100 cpm. With 48.4 being approximately 116% of 41.7 (our ideal conveyor speed), the pitch in the timing screw is 116% of our ideal pitch. The pitch in this condition is 5.8" (or 116% of 5").
EXTREME FAST OR SLOW SPEEDS:
If you are experiencing a container falling forward or backward within the screw, it is very likely the conveyor speed is much too fast or slow. Running at extreme speeds in either direction will often cause the container to fall.
JAMS AT THE DISCHARGE:
If you are experiencing jams at the discharge, this could be because with the conveyor speed not pulling the container out, the last container is stuck in the screw. Use a tachometer to understand the relationship and alter your conveyor speed.
In short, proper timing between conveyor and screw speeds helps maintain pitch and therefore helps maintain accuracy and efficiency of your other packaging equipment. This is why Morrison provides a timing screw set up sheet with our systems in our user manuals. These sheets will provide the screw and conveyor speeds we’ve deemed best for accurate operation of the intended timing screw application.
If you need help locating it, you can contact our Service Department here, or fill out a request for a new user manual. Our experts know this is one of the most confusing aspects of container handling, which is why we are here to help. Call us if you need help troubleshooting and diagnosing any feed screw problem, even if it is from a different manufacturer.
