A conveyor turnover or twist is usually found on long overland conveyor systems where one turnover is located on the return side behind the drive and the take-up, and the other turnover is located in front of the tail pulley. This configuration is the most common use of the turnover system. A single turnover can also be found in unusual applications. These will be discussed later.
The dual turnover system means that the carry side of the conveyor belt is returned "dirty side up" rather than "dirty side down" and in contact with the return idlers. This means the pulley cover side, or clean side, is in contact with the carry idlers and the return idlers also.
The dual system offers a variety of benefits to the conveyor operator such as:
1. With build up on the carry cover you reduce the surface wear on the return conveyor idlers.
2. Cover wear on the carry side is reduced because the dirty side is not being pulled across the return idlers.
3. You eliminate the build-up of frozen or sticky material on the return idlers.
4. Carry back or dribble under the conveyor is concentrated between the terminal pulley and the turnovers rather than the entire length of the return side.
Design of the turnover system is critical. The length of the twist must be long enough to prevent excess edge stresses and long enough to prevent the center of the belt from buckling. Generally, the turnovers should be on the return side of the conveyor because tensions are lower there.
A general rule of thumb for fabric conveyor belts is that the length of a 180° twist should be at a minimum of 1 foot of twist length per 1 inch of belt width. Twist lengths shorter than this can result in tensions at the belt center being too low and can cause instability in the twist area.
The belt tensions must be calculated at the point where the turnovers are located. Tensions such as running tension, acceleration tension, stopping tension and breakaway tension should be calculated in addition to other factors like belt weight, belt rated tension and belt modulus.
If the calculated tensions in the twist area are quite low, which will result in an exceptionally long twist length to prevent center buckling, additional counterweight can be added to the system to shorten the twist length. The addition of counterweight or slack side tension must be done with caution so as to not exceed the rated working tension of the belt.
The belt sag between the supporting pulleys could become large enough to affect the tensions in the belt at the turnover area. We suggest that sag be held to a maximum of 2% of the twist length.
Sag can be calculated as follows:
SAG (feet) = (Wb) (LT)2/ 8 (TT)
Where Wb = belt weight in Ibs/ft
LT = horizontal length of twist in feet
TT = tension at twist in pounds
The actual amount of belt sag will be approximately 70% of the above-calculated value due to stiffness in the vertical position at the center of the turnover.
It is also suggested that additional vertical clearance be allowed for the middle of the twist area to allow for changes in tensions that occur during acceleration and deceleration that can cause the belt to jump.
The following are also recommended for all turnover systems:
1. When using a dual turnover system, the twists should be in the same direction to balance out any edge stress that occurs
2. Automatic take-ups should be used to maintain a constant tension in the twist area
3. Controlled acceleration and deceleration
4. Vertical rolls mid-way in the twist that the belt passes through should be off-set a few inches and adjustable in all directions for containment and tracking purposes
5. The horizontal end pulleys should be adjustable for tracking purposes
6. The two twist pulleys should be 6" wider than the belt and set for a few degrees of belt wrap
The single turnover, or twist system, is constructed in the same manner as the dual system except that there is only one 180° twist in the belt.
This means that the conveying run is alternated on the carry cover and then the pulley cover and so on. The single twist system offers the conveyor operator a different set of benefits than what is found with the double twist system.
This system would be used conveying hot materials that will allow the belt conveyor to have additional cooling time as the load is alternated from one cover to the other.
Some products have an adverse reaction when exposed to rubber covers. Alumina is a typical example when conveyed over Grade II covers. The Alumina attacks the cover by extracting the plasticizers causing the rubber to get hard, crack and cup. The single pass turnover when using RMA Grade II covers will allow the Alumina to attack both sides of the belt at once which will offset the cupping problem.
Some products are highly abrasive and by using a single pass twist in the belt, both sides of the belt will be worn away rather than the carry side only. This will extend normal belt life.
All rubber covers when exposed to a single pass turnover system should be purchased as balanced covers; Example: 1/8 x 1/8, 3/16 x 3/16, 1/4 x 1/4.
The design of the single turnover system is concurrent to the double system and the same rules will apply as to turnover lengths, sag amounts and roll layout.
Contact your local SKE Industries representative for assistance with your questions concerning conveyor belt turnovers.