🔹 1. Introduction

Holdbacks (Backstops) are mechanical safety devices used on inclined or overland conveyors to prevent reverse motion of the loaded belt during power failure. They play a crucial role in ensuring the safety of men and machinery in mines.

                🔹 2. Function of Holdback

When a conveyor stops due to power loss, the loaded belt tends to move backward under gravity. The holdback locks the pulley shaft to prevent reverse rotation, avoiding belt damage, spillage, or accidents.

                   🔹 3. Types of Holdbacks

Type	Description	Remarks
Ratchet & Pawl	Mechanical stop; allows motion in one direction	Prone to wear
Differential Hand Brake	Friction-based stop system	Needs adjustment
Overrunning Clutch (Roller Type)	Uses rollers on inclined planes	Smooth operation
Overrunning Clutch (Sprag Type)	Uses sprags for torque transmission	Precise & durable

✅ Modern conveyors use Sprag-type overrunning clutches for high reliability.

🔹 4. Installation Location

• Usually on low-speed drive pulley shaft opposite to the drive.
  
• Ensures positive control over belt reversal.
  
• Alternative: Reducer shaft, but increases vibration & maintenance.
  

🔹 5. Torque & Size Selection

Backstops are selected based on:

1. Motor breakdown torque
   
2. Speed of drive pulley
   
3. Service factor (1.0–1.5)
   
4. Dual or tandem drive systems require 70% torque sharing per backstop.
   

✅ Formula:Torque (Nm)=Motor kW×9543×S.F.RPM\text{Torque (Nm)} = \frac{Motor\,kW × 9543 × S.F.}{RPM}Torque (Nm)=RPMMotorkW×9543×S.F.

🧠 25 DGMS-Style MCQs – Conveyor Holdbacks (Backstops)

1️⃣ The main function of a conveyor holdback is to —

a) Increase conveyor speed
b) Prevent belt reversal on power failure
c) Adjust belt tension
d) Control belt speed
e) Support the belt
✅ Answer: b) Prevent belt reversal on power failure
2️⃣ Holdbacks are essential in —

a) Horizontal conveyors
b) Inclined conveyors
c) Vertical shafts
d) Shuttle conveyors
e) None of these
✅ Answer: b) Inclined conveyors
3️⃣ Another term used for a holdback is —a) Brake
b) Backstop
c) Pulley lock
d) Speed controller
e) Coupling
✅ Answer: b) Backstop
4️⃣ The main safety function of a holdback is to —

a) Prevent belt sag
b) Avoid reverse motion and accidents
c) Maintain belt alignment
d) Reduce friction losses
✅ Answer: b) Avoid reverse motion and accidents
5️⃣ The simplest mechanical type of holdback is —

a) Sprag clutch
b) Ratchet and pawl
c) Differential brake
d) Roller clutch
e) Hydraulic lock
✅ Answer: b) Ratchet and pawl
6️⃣ The most advanced and reliable type of backstop used today is —

a) Ratchet and pawl
b) Differential brake
c) Overrunning clutch (Sprag type)
d) Gear lock
e) Belt friction clutch
✅ Answer: c) Overrunning clutch (Sprag type)
7️⃣ The roller type backstop transmits torque by —

a) Friction only
b) Wedge action of rollers
c) Hydraulic pressure
d) Magnetic coupling
✅ Answer: b) Wedge action of rollers
8️⃣ Sprag-type backstop consists of —

a) Cylindrical rollers
b) Irregularly shaped wedging elements
c) Springs only
d) Friction pads
✅ Answer: b) Irregularly shaped wedging elements
9️⃣ A low-speed backstop is mounted on 

a) Motor shaft
b) Drive pulley shaft (head pulley)
c) Tail pulley
d) Coupling
✅ Answer: b) Drive pulley shaft (head pulley)
🔟 A backstop must be installed to withstand —a) Average torque
b) Maximum breakdown torque of the motor
c) Rated torque
d) Starting torque only
✅ Answer: b) Maximum breakdown torque of the motor
11️⃣ In dual-drive conveyors, load sharing between backstops should be at least —

a) 30%
b) 50%
c) 70%
d) 100%
✅ Answer: c) 70%
12️⃣ The formula for calculating torque (Nm) for backstop selection is —

a) Torque = Power × 60 / RPM
b) Torque = (kW × 9543 × S.F.) / RPM
c) Torque = kW × RPM × 9.81
d) Torque = (HP × 746) / RPM
✅ Answer: b) (kW × 9543 × S.F.) / RPM
13️⃣ Service factor (S.F.) for 175% breakdown torque is —

a) 1.00
b) 1.15
c) 1.30
d) 1.50
✅ Answer: a) 1.00
14️⃣ For 225% breakdown torque, service factor a) 1.0
b) 1.15
c) 1.30
d) 1.50
✅ Answer: c) 1.30
15️⃣ The best location for a backstop installation is —

a) On reducer shaft
b) On drive pulley shaft opposite the motor
c) On motor shaft
d) On coupling
✅ Answer: b) On drive pulley shaft opposite the motor
16️⃣ If two backstops are installed on one shaft, they must —

a) Share load equally
b) Each be sized for 70% torque
c) Operate alternately
d) Not operate simultaneously
✅ Answer: b) Each be sized for 70% torque
17️⃣ For tandem drive conveyors, the backstop on the primary pulley must be sized for —

a) Primary drive torque only
b) Total torque of primary and secondary drives
c) Secondary drive torque
✅ Answer: b) Total torque of primary and secondary drives
18️⃣ A backstop freewheels when —

a) Conveyor runs forward
b) Conveyor reverses
c) Motor is off
d) Belt is loaded
✅ Answer: a) Conveyor runs forward
19️⃣ The major advantage of an overrunning clutch type backstop is —

a) No backlash, automatic engagement
b) High wear
c) Requires lubrication often
d) High stress concentration
✅ Answer: a) No backlash, automatic engagement
20️⃣ The wear in a sprag clutch occurs due to —a) Constant rubbing of sprags during freewheel
b) Excess lubrication
c) Shaft vibration only
✅ Answer: a) Constant rubbing of sprags during freewheel
21️⃣ If a conveyor reverses without a backstop, it can cause —

a) Spillage of material
b) Injury or death
c) Belt tear or destruction
d) All of the above
✅ Answer: d) All of the above
22️⃣ When a low-speed backstop is mounted on a reducer shaft, it is exposed to —

a) Higher vibration and heat
b) Lower torque
c) No load
✅ Answer: a) Higher vibration and heat
23️⃣ Typical speed of a low-speed backstop equals —

a) Motor RPM
b) Drive pulley RPM
c) Belt RPM
d) Reducer input RPM
✅ Answer: b) Drive pulley RPM
24️⃣ Dual tandem drives require —

a) Two backstops on primary shaft
b) One backstop per drive
c) Both a & b depending on load
✅ Answer: c) Both a & b depending on load
25️⃣ DGMS classifies conveyor backstops as —a) Power transmission devices
b) Safety devices
c) Control equipment
d) Speed regulators
✅ Answer: b) Safety devices

🧾 Quick Recap

• Purpose: Prevent belt reversal & accidents
  
• Types: Ratchet, Differential, Roller, Sprag
  
• Best Type: Sprag-type overrunning clutch
  
• Torque Formula: (kW × 9543 × S.F.) / RPM
  
• Safety: Each backstop rated for 70% load sharing
  

                        🔚 7. Conclusion

Holdbacks are vital safety devices in conveyor systems, preventing catastrophic rollbacks and protecting both machinery and workers. Proper selection based on torque and motor capacity ensures long service life and safe operation.

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