NTN Auto Tensioners

Introduction

Auto Tensioners have been widely adopted for automobile engines since the 1980s. With the recent trend towards high-performance automobile engines, they are used primarily for improved reliability and low noise operation of the timing belts used extensively in camshaft drives. Today, Auto Tensioners are installed chiefly on engines of 2 liters and greater, and the mainstream is the hydraulic type Auto Tensioner with its superb performance.

Along with the recent demands for cost reduction, small-size engines also require higher reliability. Other major trends include a shift from timing belts to timing chains and the use of serpentine drives that require only a single accessory drive belt. Auto Tensioners effectively improve the reliability and reduce the noise of these timing belts and chains.

NTN currently offers a variety of types of hydraulic Auto Tensioners for timing belts. To meet the needs of the market, NTN has a product line based on a hydraulic Auto Tensioner with its excellent damping performance, such as cost reduction and compact Auto Tensioners, integrated pulley arm Auto Tensioner units, and built-in pulley type Auto Tensioner units. In addition to Auto Tensioners for timing belts, NTN also offers Auto Tensioners for timing chains (for automobiles and motorcycles) and Auto Tensioners for accessory drive belts that are needed for serpentine drives.

i. What is an Auto Tensioner?

Auto Tensioner is an abbreviation of Automatic Tensioner and is a device that constantly maintains proper tension of the timing belts or chains that transfer the crankshaft rotation to the camshaft, and of the accessory drive belts. The timing belts and chains use gear drive whereas the accessory drive belts use friction drive.

ii. Why do you need an Auto Tensioner?

An Auto Tensioner absorbs changes in or loss of tension of the timing belts and chains caused by changes in engine temperatures or speed, or by aging of those materials, and maintains the tension at a constant level, thus reducing noise and improving the service life.

A graph illustrates the tension adjustment function with respect to temperature changes. It shows that as engine temperature increases, the displacement in the tension pulley and the force applied to the auto tensioner also change, indicating the tensioner's ability to maintain optimal tension.

iii. Types of NTN Auto Tensioners

Hydraulic type Auto Tensioners are excellent in resisting aging and performing tension adjustment to temperature changes, which are required of all auto tensioners. All NTN Auto Tensioners apply the hydraulic type. NTN Auto Tensioners come in three types: for Timing Belt, Timing Chain, and Accessory Drive Belt.

A diagram shows the classification of NTN Auto Tensioners, categorizing them into types for Timing Chain (Buttress Thread Type, Ring Type), Timing Belt (Basic type, Size Reduction/Short type, Ultra compact type, Integrated pulley arm type, Built-in pulley type), and Accessory Drive Belt (Separate type, Standard type, Compact type).

iv. How is an Auto Tensioner different from a fixed type tensioner?

Both fixed type and automatic tensioners are used to provide tension needed for transmitting driving force. When a fixed type tensioner is used, it is necessary to set the initial belt tension higher than the optimal value so that the belt will not slip in the event of belt stretch or wear. If the initial tension is set excessively higher than the optimal level, the belt may sustain great deformation, which can lead to loss of transmission efficiency or service life.

On the other hand, an Auto Tensioner ensures proper transmission efficiency throughout the entire service life of the belt, allowing little changes in belt tension despite stretches or wear. A comparison graph shows that Auto Tensioners improve the timing belt service life compared to fixed type tensioners.

For Timing Belt

The document details various types of Auto Tensioners for timing belts, including:

• Basic type Auto Tensioner: Features lightweight and wear resistance through aluminum alloy cylinders and steel valve sleeves, reduced parts due to a gas-liquid 2-phase structure, and improved moment load resistance via a 2-point support structure for the rod. Specifications include an overall length of 85mm min., rod stroke of 18mm max., and rod force of 250N max.
• Short type Auto Tensioner: Achieves a 30% reduction in overall length compared to the basic type due to the parallel layout of the valve sleeve and return spring, offering similar performance. Specifications include an overall length of 60mm min., rod stroke of 18mm max., and rod force of 300N max.
• Ultra compact type Auto Tensioner: Offers a 30% weight and size reduction compared to the basic type, with similar performance. Specifications include an overall length of 60mm min., rod stroke of 14mm max., and rod force of 210N max.
• Integrated pulley arm type Auto Tensioner unit: Integrates the cylinder and bracket through co-molding, with the entire unit oscillating around a pivot bearing to adjust tension. It is lighter, more compact, and more cost-effective than separate type models, but requires a rod contact pin on the engine side.
• Built-in pulley type Auto Tensioner unit: Integrates an ultra-compact hydraulic damper and return spring into a pulley. This unit is compact and cost-effective, designed for small in-line 4-cylinder engines. It can replace a fixed type pulley but requires a location pin on the engine side. Specifications include pulley OD of 65mm min., pulley center height of 24.5mm min., rod stroke of 9mm max., and pulley force of 180N max.

A performance comparison table and diagrams illustrate the features and specifications of these types.

Principles of Operation

1. Preventing belts from flapping: When the belt is tight, the rod is pushed in, closing the check valve. The belt load is supported by the hydraulic damper. When the belt is loose, the return spring pushes the rod out, moving the pulley against the belt to prevent flapping.

2. Adjusting belt tension: An Auto Tensioner moves the pulley to appropriate positions in accordance with changes in belt tension (due to temperature changes or elongation), thus maintaining constant belt tension.

For Timing Chain

NTN offers two types of Auto Tensioners for timing chains:

• Buttress Thread type Chain Tensioner: Features a high-performance, stepless No-back mechanism utilizing buttress threads. This mechanism locks the threads when the engine is not running, limiting plunger movement and reducing noise during low oil pressure. When the engine is running, the threads do not lock, providing excellent tension adjustment. Aluminum cylinders are used for weight reduction. The No-back function is stepless, ensuring constant backlash, while the tension adjusting function uses a hydraulic damper with optimal adjustment via a relief valve. It also includes an over-tension preventive function with dynamic push-in characteristics. Specifications include a plunger stroke of 20mm max., plunger diameter of ø16mm, and return spring force of 60N max.
• Ring type Chain Tensioner: Achieves low cost by adopting a simple No-back mechanism using a ring, which provides initial setting, no-back, and anti-disassembly functions. Aluminum cylinders are used for weight reduction. Specifications include a plunger stroke of 20mm max., plunger diameter of ø16mm, and return spring force of 60N max.

The document also details the application of Ring type Chain Tensioners for motorcycles, highlighting their tuned performance for motorcycle engines, including leakage gap, relief valve opening pressure, and return spring loading.

The operation of the tensioner is explained, including the initial setting release mechanism. When installing on engines, the plunger is pushed out by the return spring, and the pin and holding device are locked for initial setting. When releasing the initial setting, the chain guide pushes the plunger as the engine starter motor runs, releasing the holding device and pin.

For Accessory Drive Belt (serpentine)

Serpentine drives use a single long belt to drive accessory devices and are becoming more popular for their benefits in engine design and serviceability. As accessory loads increase, NTN's Auto Tensioners ensure constant belt tension, contributing to extended belt service lives.

Characteristics:

• An external return spring provides higher belt tension.
• Ideal for 4-cycle and diesel engines with significant belt tension fluctuations.

Specifications:

• Standard type: Size ø45 x 129, Rod stroke 18mm max., Spring force 1200N (at center of stroke), Rated loading 2500N.
• Compact type: Size ø34 x 108.5, Rod stroke 16.5mm max., Spring force 1000N (at center of stroke), Rated loading 2000N.

Diagrams illustrate the standard and compact types, as well as a unit configuration for accessory drive belts.

Actual production line-up

The document showcases a wide range of NTN Auto Tensioners applied to various engine types, including L4, V6, V8, and L6 engines, as well as diesel engines. These examples demonstrate the versatility and widespread adoption of NTN's auto tensioner technology across different automotive applications.

References / Literature

• Practical designing for belt drives, Yokendo Publishing
• Designing safety in roller chains, Yokendo Publishing
• Articles about technologies of automobile manufacturers, etc.