T2 (SLACK SIDE TENSION) (T2)
T2 is the minimum belt tension on the slack side of the drive pulley; it must exceed Te / (e^(μθ) − 1) per Eytelwein to prevent drive slip, and it is set in the field by the take-up.
T2, the slack-side tension, is the belt tension immediately leaving the drive pulley on the return side — the smallest steady-state belt tension in the entire loop. It is established physically by the [take-up pulley](/glossary/take-up-pulley): the gravity weight or screw load on the take-up is what pulls T2 up to the required magnitude. The drive pulley then adds Te to T2 around its wrap to give T1 on the entry side: T1 = T2 + Te.
The minimum value of T2 is governed by the [Eytelwein equation](/glossary/capstan-equation-eytelwein). To transmit a given Te without belt slip, T2 ≥ Te / (e^(μθ) − 1), where μ is the belt-to-pulley friction coefficient (set by lagging) and θ is the wrap angle in radians. A 180° wrap with rubber lagging at μ = 0.35 requires T2 ≥ Te / 2.0; a 230° wrap with ceramic lagging at μ = 0.45 needs only T2 ≥ Te / 5.2. Designers add a slip safety factor of 1.3–1.5 to this minimum.
On long conveyors, T2 may instead be governed by sag — the minimum tension required to keep return-side sag below 2–3 % of idler spacing. On heavy steel-cord overland belts this 'sag T2' often exceeds the 'slip T2', so it becomes the controlling value. T2 is also raised during start-up by the take-up dynamic response, which the take-up sizing must accommodate. The [belt tension calculator](/tools/conveyor-belt-tension-calculator) returns both T1 and T2 for a given duty.
Formula
T2 ≥ Te / (e^(μθ) − 1) (drive slip threshold)
| Symbol | Meaning | Unit |
|---|---|---|
| T2 | Slack-side belt tension at the drive pulley | N |
| Te | Effective tension across the drive | N |
| μ | Belt-to-pulley friction coefficient | — |
| θ | Wrap angle of belt on the drive pulley | rad |
Related engineering tools
Related terms
- Effective Tension (Te)(Te)
Effective tension (Te) is the net tangential force that the drive pulley must transmit to the belt to overcome all motion resistances; it is the fundamental input for motor power and belt selection.
- T1 (Tight Side Tension)(T1)
T1 is the maximum belt tension at the drive pulley entry — the sum of slack-side tension T2 plus effective tension Te — and is the value used to size belt rating in kN per metre of belt width.
- Capstan Equation (Eytelwein)
The Capstan or Eytelwein equation T1/T2 = e^(μθ) describes the maximum tension ratio a belt can sustain across a driven pulley before slipping, given the friction coefficient μ and the wrap angle θ in radians.
- Take-up Pulley
A take-up pulley sits on the return strand and is moved by a gravity weight or a screw mechanism to apply the slack-side tension T2, absorbing belt elongation and preventing sag and slip.
- Belt Sag
Belt sag is the vertical drop of a conveyor belt between two adjacent idlers under the combined weight of belt and load, typically kept below 1.5 % of idler spacing on the carry side.