In process lines, tension control is an important aspect of line operation. A Bridle roll unit allows for the control of tension in various areas of the line, with each section (divided by the bridles) able to maintain a different tension.
Bridles also compensate for tension lost throughout the line as a result of bending the strip around the rolls (bending losses).
A Bridle roll unit can affect the tension in the strip according to the formula T2 = T1·eμα, where T1 and T2 are the tensions on either side of the bridle roll, μ is the coefficient of friction between the roll and the strip, and α is the angle of wrap of the strip around the roll in radians.
So, the greater the coefficient of friction and angle of wrap, the more effect each bridle roll can have on the strip tension.
In order to achieve this tension differential, the bridle rolls must also be driven, otherwise they will have little effect.
The power required for each roll is according to the formula P = ΔT·vstrip, where P is the power required in W, ΔT is the difference in tensions in N, and vstrip is the line speed in m/s.
Snubber rolls can also be applied to add tension to either side of the bridle unit. The tension added is according to Tp = c·P·μ, where Tp is the tension added, P is the force applied by the snubber roll, and μ is the coefficient of friction.
The coefficient c is equal to 1 if the snubber roll is not driven and 2 if it is driven.
Diameter of bridle rolls – Larger rolls are more expensive, but cause less bending of the strip. Thicker strip usually requires a larger roll diameter to reduce the stress caused by wrapping around the rolls.
Roll diameter also depends on the line section. For instance, larger rolls are normally used after process section. (e.g. Entry – Ø800, Intermediate – Ø1250, Exit – Ø1000 / Ø1250) Rolls are generally no smaller than Ø800 and can be up to Ø1800 for heavier lines.
Length of bridle rolls – Dependant on the maximum strip width. Generally, use the strip width plus some room for deviation (strip width + approx. 300).
Material of bridle rolls – Standard: use tungsten carbide coated rolls if they are driven and chrome coated rolls if they are not driven.
Tungsten carbide has relatively good wear resistance and a good coefficient of friction with steel, which improves tension. For surface sensitive strip (as in pickling lines), use PU coating (also rubber) whether the rolls are driven or not.
Shape of bridle rolls – Standard: use a crowned profile for chrome and PU coated rolls, and a cylindrical profile for tungsten carbide coated rolls. Crowned rolls improve strip tracking, but not predictably.
Bearing diameter on rolls – The bearing diameter must be large enough to withstand the load on the roll due to the strip tension. The bearing must also be large enough to accommodate the roll shaft which is sized according to the input torque from the drives.
Sizes are determined through drive calculation. Standard sizes are Ø140 for light duty, Ø190 for medium duty, and Ø280 for heavy duty.
Roll configuration – Standard: 2-roll ‘flat’ configuration (either direction) as shown.
Though there are several other ways to arrange the bridle rolls, these are the simplest and most efficient because of the large angle of wrap. This configuration also provides relatively good roll access.
For each bridle roll diameter, there are standard vertical and horizontal distances between the roll centers.
Often the incoming or outgoing strip will not be horizontal (which means less strip wrap), or the strip may be going the opposite direction than what is shown here, but the roll configuration remains standard.
In some cases, two bridle roll units are paired up to form a four roll bridle, providing more tension control in one place. Occasionally a non-standard roll configuration will be used as well.
Roll drives – Whether a roll is driven or not and how much power the drive needs depends on the tension differentials between each tension section of the line.
In general, this is calculated using drive calculation program. The program also calculates the gear ratios and motor speeds required.
Snubber rolls – Snubber rolls applied to the bridle rolls can help increase tension in the strip and help prevent slipping of the strip, especially where there is a large tension differential between sections, or tension in one of the sections might be 0 (e.g. after the welder).
Snubber rolls are cylindrical and PU coated, and may be one of two types:
- Pinch rolls may be driven or non-driven and are hydraulically actuated, Ø300, and the same length as the bridle roll.
- Pressure rolls are non-driven and are pneumatically actuated, Ø200, and 500 wide.
There are different designs for snubber roll sub-assemblies depending on where on the bridle the roll is applied and how much nip force is required (required nip force is calculated by the drive calculation program).
Mounting – Standard: the bridle unit and drive base are mounted on the concrete foundation. Depending on the line layout, some units may be mounted on steel structure.
Polishing Devices – Polishing devices are used to clean the rolls as they transport the strip. Generally, they are used on rolls that are after the zinc pot or skin pass mill in a line.
Polishing devices use counterweights to keep them engaged and may be manually disengaged, or pneumatically disengaged. Some polishing devices are designed to oscillate across the roll face using a small motor.To return from this bridle roll unit page to the Strip Metal Coil Processing Home page, click here.
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